SOS1 inhibition enhances the efficacy of and delays resistance to G12C inhibitors in lung adenocarcinoma

Clinical effectiveness of KRAS G12C inhibitors (G12Cis) is limited both by intrinsic and acquired resistance, necessitating the development of combination approaches. We found that targeting proximal receptor tyrosine kinase (RTK) signaling using the SOS1 inhibitor (SOS1i) BI-3406 both enhanced the potency of and delayed resistance to G12Ci treatment, but the extent of SOS1i effectiveness was modulated by both SOS2 expression and the specific mutational landscape. SOS1i enhanced the efficacy of G12Ci and limited rebound RTK/ERK signaling to overcome intrinsic/adaptive resistance, but this effect was modulated by SOS2 protein levels. Survival of drug-tolerant persister (DTP) cells within the heterogeneous tumor population and/or acquired mutations that reactivate RTK/RAS signaling can lead to outgrowth of tumor initiating cells (TICs) that drive therapeutic resistance. G12Ci drug tolerant persister cells showed a 2-3-fold enrichment of TICs, suggesting that these could be a sanctuary population of G12Ci resistant cells. SOS1i re-sensitized DTPs to G12Ci and inhibited G12C-induced TIC enrichment. Co-mutation of the tumor suppressor KEAP1 limits the clinical effectiveness of G12Cis, and KEAP1 and STK11 deletion increased TIC frequency and accelerated the development of acquired resistance to G12Ci in situ. SOS1i both delayed acquired G12Ci resistance and limited the total number of resistant colonies regardless of KEAP1 and STK11 mutational status. These data suggest that SOS1i could be an effective strategy to both enhance G12Ci efficacy and prevent G12Ci resistance regardless of co-mutations.

Discovery of BI 7446: A Potent Cyclic Dinucleotide STING Agonist with Broad-Spectrum Variant Activity for the Treatment of Cancer

Activating the stimulator of interferon genes (STING) pathway with STING agonists is an attractive immune oncology concept to treat patients with tumors that are refractory to single-agent anti-PD-1 therapy. For best clinical translatability and broad application to cancer patients, STING agonists with potent cellular activation of all STING variants are desired. Novel cyclic dinucleotide (CDN)-based selective STING agonists were designed and synthesized comprising noncanonical nucleobase, ribose, and phosphorothioate moieties. This strategy led to the discovery of 2',3'-CDN 13 (BI 7446), which features unprecedented potency and activates all five STING variants in cellular assays. ADME profiling revealed that CDN 13 has attractive drug-like properties for development as an intratumoral agent. Injection of low doses of CDN 13 into tumors in mice induced long-lasting, tumor-specific immune-mediated tumor rejection. Based on its compelling preclinical profile, BI 7446 has been advanced to clinical trials (monotherapy and in combination with anti-PD-1 antibody).

Brain areas activated during visual learning in the cichlid fish Pseudotropheus zebra

The neural correlates of most cognitive functions in fish are unknown. This project aimed to identify brain regions involved in visual learning in the cichlid fish Pseudotropheus zebra. The expression of the protein pS6 was measured in 19 brain areas and compared between groups of individuals subjected to four different behavioral contexts (control, avoidance, trained, and novelty groups). Control group individuals were sacrificed with minimal interactions. Fish in the avoidance group were chased with a net for an hour, after which they were sacrificed. Individuals in the trained group received daily training sessions to associate a visual object with a food reward. They were sacrificed the day they reached learning criterion. Fish in the novelty group were habituated to one set of visual stimuli, then faced a change in stimulus type (novelty stimulus) before they were sacrificed. Fish in the three treatment groups showed the largest activation of pS6 in the inferior lobes and the tectum opticum compared to the control group. The avoidance group showed additional activation in the preoptic area, several telencephalic regions, the torus semicircularis, and the reticular formation. The trained group that received a food reward, showed additional activation of the torus lateralis, a tertiary gustatory center. The only area that showed strong activation in all three treatment groups was the nucleus diffusus situated within the inferior lobe. The inferior lobe receives prominent visual input from the tectum via the nucleus glomerulosus but so far, nothing is known about the functional details of this pathway. Our study showed for the first time that the inferior lobes play an important role in visual learning and object recognition.

Pharmacodynamic and Antitumor Activity of BI 836880, a Dual Vascular Endothelial Growth Factor and Angiopoietin 2 Inhibitor, Alone and Combined with Programmed Cell Death Protein-1 Inhibition

Vascular endothelial growth factor (VEGF) and angiopoietin (ANG)-2 have complementary roles in angiogenesis and promote an immunosuppressive tumor microenvironment. It is anticipated that the combination of VEGF and ANG2 blockade could provide superior activity to the blockade of either pathway alone and that the addition of VEGF/ANG2 inhibition to an anti-programmed cell death protein-1 (PD-1) antibody could change the tumor microenvironment to support T-cell-mediated tumor cytotoxicity. Here, we describe the pharmacologic and antitumor activity of BI 836880, a humanized bispecific nanobody comprising two single-variable domains blocking VEGF and ANG2, and an additional module for half-life extension in vivo. BI 836880 demonstrated high affinity and selectivity for human VEGF-A and ANG2, resulting in inhibition of the downstream signaling of VEGF/ANG2 and a decrease in endothelial cell proliferation and survival. In vivo, BI 836880 exhibited significant antitumor activity in all patient-derived xenograft models tested, showing significantly greater tumor growth inhibition (TGI) than bevacizumab (VEGF inhibition) and AMG386 (ANG1/2 inhibition) in a range of models. In a Lewis lung carcinoma syngeneic tumor model, the combination of PD-1 inhibition with VEGF inhibition showed superior efficacy versus the blockade of either pathway alone. TGI was further increased with the addition of ANG2 inhibition to VEGF/PD-1 blockade. VEGF/ANG2 inhibition had a strong antiangiogenic effect. Our data suggest that the blockade of VEGF and ANG2 with BI 836880 may offer improved antitumor activity versus the blockade of either pathway alone and that combining VEGF/ANG2 inhibition with PD-1 blockade can further enhance antitumor effects. SIGNIFICANCE STATEMENT: Vascular endothelial growth factor (VEGF) and angiopoietin (ANG)-2 play key roles in angiogenesis and have an immunosuppressive effect in the tumor microenvironment. This study shows that BI 836880, a bispecific nanobody targeting VEGF and ANG2, demonstrates substantial antitumor activity in preclinical models. Combining VEGF/ANG2 inhibition with the blockade of the PD-1 pathway can further improve antitumor activity.

Combined KRASG12C and SOS1 inhibition enhances and extends the anti-tumor response in KRASG12C-driven cancers by addressing intrinsic and acquired resistance

Efforts to improve the anti-tumor response to KRASG12C targeted therapy have benefited from leveraging combination approaches. Here, we compare the anti-tumor response induced by the SOS1-KRAS interaction inhibitor, BI-3406, combined with a KRASG12C inhibitor (KRASG12Ci) to those induced by KRASG12Ci alone or combined with SHP2 or EGFR inhibitors. In lung cancer and colorectal cancer (CRC) models, BI-3406 plus KRASG12Ci induces an anti-tumor response stronger than that observed with KRASG12Ci alone and comparable to those by the other combinations. This enhanced anti-tumor response is associated with a stronger and extended suppression of RAS-MAPK signaling. Importantly, BI-3406 plus KRASG12Ci treatment delays the emergence of acquired adagrasib resistance in both CRC and lung cancer models and is associated with re-establishment of anti-proliferative activity in KRASG12Ci-resistant CRC models. Our findings position KRASG12C plus SOS1 inhibition therapy as a promising strategy for treating both KRASG12C-mutated tumors as well as for addressing acquired resistance to KRASG12Ci.

Abstract 2667: Trial in progress: Phase 1 study of BI 1823911, an irreversible KRASG12C inhibitor targeting KRAS in its GDP-loaded state, as monotherapy and in combination with the pan-KRAS SOS1 inhibitor BI 1701963 in solid tumors expressing KRASG12C mutation

Inhibition of KRASG12C mediated signaling and the therapeutic impact in non-small cell lung cancer (NSCLC) whose tumors carry this mutation was demonstrated clinically by sotorasib and adagrasib leading to approval of sotorasib in KRASG12C mutant NSCLC. These encouraging data are currently changing the treatment paradigm for patients with KRASG12C-mutated tumors. However only a fraction of patients is initially responding to these compounds and patients who achieved an objective response ultimately progressed on-treatment. It became clear from these studies that KRASG12C inhibitors require a combination partner to either achieve a deeper response initially or to prevent development of resistance. Multiple rational combination approaches are currently investigated with the goal to prolong duration of response or to overcome KRASG12C inhibitor resistance. The KRASG12C inhibitor BI 1823911 is more potent compared to sotorasib or adagrasib and showed comparable in vivo efficacy at a dose of 60 mg/kg vs. 100 mg/kg of either sotorasib or adagrasib in preclinical studies. The pan-KRAS SOS1 inhibitor BI 1701963 is the first direct KRAS signaling modifier, which entered phase I clinical trials both as a monotherapy as well as in combination with KRASG12C inhibitors, MEK inhibitors and irinotecan. Pan-KRAS SOS1 inhibitors exhibit activity against a broad spectrum of KRAS alleles, including the major G12D/V/C and G13D oncoproteins, while sparing the interaction of KRAS with SOS2. In the presented combination concept BI 1701963 shifts the balance of KRASG12C to its GDP-loaded form, which is the state to which BI 1823911 covalently binds to. Here, we present pre-clinical data showing enhanced pathway modulation and synergistic anti-tumor effects following vertical pathway inhibition of BI 1823911 in combination with BI 1701963. Our preclinical results supported the start of a phase I trial (NCT04973163), investigating the safety, tolerability, recommended dose and preliminary efficacy of BI 1823911 alone and in combination with the pan-KRAS SOS1 inhibitor BI 1701963. The trial includes cohorts of patients with KRASG12C mutant solid tumors, such as NSCLC, CRC, cholangiocarcinoma, and pancreatic adenocarcinoma, both KRAS therapy naïve or KRAS therapy relapsed. The first patients in the trial were treated in the monotherapy arm, dose escalation started at a dose of 50 mg. The combination therapy part is expected to start in Q1 2022. Primary endpoints include dose-limiting toxicities, treatment-emergent or -related adverse events. Secondary endpoints include pharmacokinetic properties of combination regimens and preliminary efficacy.

Citation Format: Irene C. Waizenegger, Hengyu Lu, Claus Thamer, Fabio Savarese, Daniel Gerlach, Dorothea Rudolph, Christopher P. Vellano, Marcelo Marotti, John Heymach, Scott Kopetz, Timothy P. Heffernan, Joseph R. Marszalek, Mark P. Petronczki, Marco H. Hofmann, Norbert Kraut. Trial in progress: Phase 1 study of BI 1823911, an irreversible KRASG12C inhibitor targeting KRAS in its GDP-loaded state, as monotherapy and in combination with the pan-KRAS SOS1 inhibitor BI 1701963 in solid tumors expressing KRASG12C mutation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2667.

Abstract 3255: Probing and overcoming KRASG12C inhibitor resistance by combination with a pan-KRAS SOS1 inhibitor

The recent accelerated approval of the KRASG12C mutant-selective inhibitor sotorasib (AMG 510) for the treatment of 2nd line KRASG12C mutation-positive NSCLC patients marks the first approved targeted therapy for tumors with any KRAS mutation. While KRASG12C inhibitors deliver clinical benefit, most patients who achieved an objective response ultimately progressed. Recent insights into clinical KRASG12C inhibitor resistance identified reactivation of the RAS/MAPK pathway as a common putative driver mechanism of resistance. Multiple ongoing trials seek to augment responses to KRASG12C inhibitors through rational combination strategies, including the pan-KRAS SOS1 inhibitor BI 1701963. Here we use different preclinical experimental approaches to interrogate KRASG12C inhibitor resistance mechanisms with the aim to identify strategies to overcome resistance. To predict on-target resistance, Ba/F3 cells were transduced with KRASG12C, ENU-mutagenized and chronically exposed to KRASG12C inhibitors. Resistant clones were screened for secondary KRAS mutations, highlighting that KRAS G12C/Y96D and Y96S cis mutations did confer resistance to KRASG12C inhibition but could be overcome by MEK inhibitor combination with a SOS1 inhibitor. As second strategy a high-complexity single site variant library of KRASG12C encompassing all possible secondary KRAS mutations was employed to establish Ba/F3 transgenic cell pools. The response of this KRASG12C Ba/F3 clone library harboring a comprehensive set of secondary mutations was tested following treatment with KRASG12C inhibitors alone and in combination with a pan-KRAS SOS1 inhibitor. In parallel, acquired KRASG12C inhibitor resistance was generated in solid cancer cells following long-term MRTX849 treatment. Clones were characterized and their response to KRASG12C inhibition and combination therapy was analyzed. Both in the Ba/F3 cell pool as well as in KRASG12C inhibitor resistant clones combining SOS1 inhibition to KRASG12C inhibition proved to be beneficial. Finally, bringing these findings in vivo, SW837(CRC) tumor-bearing mice were long-term treated with adagrasib until tumors relapsed after initial regression and resistant tumors were randomized for second line treatments. In this KRASG12C inhibitor resistant context, treatment with adagrasib plus cetuximab resulted in tumor stasis and adagrasib plus SOS1i resulted in tumor regression. While more work is currently being undertaken to map the resistance mechanisms in both our in vitro and in vivo settings, all results highlight the benefit of combining a SOS1 inhibitor with a KRASG12C inhibitor to prevent and/or overcome acquired resistance. The pan-KRAS SOS1 inhibitor BI 1701963 is the first direct RAS signaling modifier in phase I clinical trials both as a monotherapy as well as in combination with KRASG12C inhibitors, MEK inhibitors and liposomal irinotecan.

Citation Format: Marco H. Hofmann, Sabine Jurado, Daniel Gerlach, Takamasa Koga, Francesca Trapani, Michael Gmachl, Donat Alpar, Simone Lieb, Astrid Jeschko, Tetsuya Mitsudomi, Norbert Kraut, Mark P. Petronczki. Probing and overcoming KRASG12C inhibitor resistance by combination with a pan-KRAS SOS1 inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3255.

Abstract 1271: In vitro and in vivo characterization of BI 1823911 - a novel KRASG12C selective small molecule inhibitor

KRASG12C mutations are predominantly found in non-small cell lung cancer (NSCLC, 13%), in colorectal cancer (CRC, 3%), and with a lower prevalence in pancreatic ductal adenocarcinoma (PDAC, 1%). The amino acid exchange at position 12 from glycine to cysteine renders RAS insensitive to GAP-catalyzed hydrolysis but not to intrinsic hydrolysis and consequently, KRASG12C is still dependent on GEF stimulation to achieve full activation. The active GTP-loaded form of KRASG12C is favored and leads to activation of downstream signaling and proliferation. A number of recent publications has shown that targeting this mutant form of KRAS, using several covalent KRASG12C inhibitors binding to the inactive GDP-KRASG12C form, leads to anti-proliferative effects and induction of apoptosis in KRASG12C mutant cancer cell lines, CDX and PDX models. Early clinical data for AMG 510 and MRTX849 revealed a response rate of 35-45% in NSCLC and of 7-17% in CRC patients. Here, we show that BI 1823911 has potent anti-proliferative activity in a panel of KRASG12C mutant cancer cell lines with higher or similar potency compared to these two most advanced compounds in clinical development. In a panel of KRASG12C NSCLC cell lines, treatment with BI 1823911 results in downregulation of MAPK pathway-responsive genes, such as DUSP6 and CCND1, and the extent of pathway modulation correlates with sensitivity. Likewise, we observe strong and sustained inactivation of the MAPK pathway at the protein level using p-ERK as a pharmacodynamic (PD) biomarker. A MIA PaCa-2 cell line-derived pancreatic cancer xenograft model was selected for extensive PK/PD/efficacy analyses in vivo. Briefly, BI 1823911 tested at 60 mg/kg showed similar anti-tumor activity compared to both competitor compounds dosed at clinically relevant exposures. Results of the ongoing in-depth PK/PD/efficacy analysis will be shared. Furthermore, BI 1823911 was tested with a daily oral dose of 60 mg/kg in a panel of NSCLC or CRC CDX or PDX mouse models and showed comparable efficacy to AMG 510 and MRTX849, respectively. Preclinical and clinical data suggest that monotherapy with a KRASG12C inhibitor will not be sufficient to achieve durable response. Combination therapy of a KRASG12C inhibitor may therefore lead to enhanced anti-tumor efficacy and may address adaptive resistance mechanisms. Therefore, we selected a panel of KRASG12C mutant cancer cell lines and tested a large set of compounds in combination with BI 1823911 to identify synergistic anti-proliferative activity. Among other MAPK and PI3K pathway inhibitors, a SOS1::KRAS inhibitor was confirmed as promising combination partner. We will show results from in vitro and in vivo combination studies in NSCLC and CRC tumor models that show deep and durable responses upon combination of BI 1823911 with SOS1::KRAS inhibitor BI 1701963 providing a strong rationale for clinical investigation of this combination.

Citation Format: Fabio Savarese, Andreas Gollner, Dorothea Rudolph, Jesse Lipp, Johannes Popow, Marco H. Hofmann, Heribert Arnhof, Jörg Rinnenthal, Francesca Trapani, Michael Gmachl, Daniel Gerlach, Joachim Broeker, Peter Ettmayer, Andreas Mantoulidis, Jason Phan, Christian A. Smethurst, Matthias Treu, Alex G. Waterson, Hengyu Lu, Annette Machado, Joseph Daniele, Stephan W. Fesik, Christopher P. Vellano, Timothy P. Heffernan, Joseph R. Marszalek, Darryl B. McConnell, Mark Petronczki, Norbert Kraut, Irene C. Waizenegger. In vitro and in vivo characterization of BI 1823911 - a novel KRASG12C selective small molecule inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1271.

Abstract CT210: Trial in Process: Phase 1 studies of BI 1701963, a SOS1::KRAS Inhibitor, in combination with MEK inhibitors, irreversible KRASG12C inhibitors or irinotecan

The SOS1i::KRAS inhibitor BI 1701963 is the first direct RAS signaling modifier to enter phase I clinical trials both as a monotherapy as well as in combination. SOS1::KRAS inhibitors exhibit activity against a broad spectrum of KRAS alleles, including the major G12D/V/C and G13D oncoproteins, while sparing the interaction of KRAS with SOS2. Here, we present pre-clinical data showing enhanced pathway modulation and synergistic anti-tumor effects following vertical pathway inhibition of BI 1701963 in combination with mitogen-activated protein kinase inhibitors (MEKi; trametinib and BI 3011441) or KRAS G12C inhibitors (MRTX849 and BI 1823911). Furthermore, SOS1::KRAS inhibitor treatment sensitizes tumor cells to increased DNA damage in combination with irinotecan. Our results highlight the suitability of SOS1::KRAS inhibitors as a backbone in combination therapies targeting KRAS-dependent tumors. Pre-clinical combination data supported the start of multiple phase I trials investigating the safety, tolerability, recommended dose and preliminary efficacy of BI 1701963 alone and in combination with other anti-cancer agents. Combination trials of BI 1701963 with MEKi include cohorts of patients with KRAS mutant solid tumors, such as non-small cell lung cancer (NSCLC), colorectal cancer (CRC), cholangiocarcinoma and pancreatic adenocarcinoma. Trials exploring the combination of BI 1701963 with irreversible KRAS G12C inhibitors (MRTX849 and BI 1823911) will include cohorts of patients with KRAS G12C mutant NSCLC and CRC. In a further study, the combination of BI 1701963 with irinotecan is being evaluated in patients with KRAS mutant CRC. Primary endpoints include dose-limiting toxicities, treatment-emergent or -related adverse events. Secondary endpoints include pharmacokinetic and pharmacodynamic properties of combination regimens and preliminary efficacy.

Citation Format: Marco H. Hofmann, Hengyu Lu, Ulrich Duenzinger, Daniel Gerlach, Francesca Trapani, Annette A. Machado, Joseph R. Daniele, Irene Waizenegger, Michael Gmachl, Dorothea Rudolph, Christopher P. Vellano, Marcelo Marotti, Vitomir Vucenovic, Timothy P. Heffernan, Joseph R. Marszalek, Mark P. Petronczki, Norbert Kraut. Trial in Process: Phase 1 studies of BI 1701963, a SOS1::KRAS Inhibitor, in combination with MEK inhibitors, irreversible KRASG12C inhibitors or irinotecan. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr CT210.

KRAS Secondary Mutations That Confer Acquired Resistance to KRAS G12C Inhibitors, Sotorasib and Adagrasib, and Overcoming Strategies: Insights From In Vitro Experiments

INTRODUCTION

KRAS mutations have been recognized as undruggable for many years. Recently, novel KRAS G12C inhibitors, such as sotorasib and adagrasib, are being developed in clinical trials and have revealed promising results in metastatic NSCLC. Nevertheless, it is strongly anticipated that acquired resistance will limit their clinical use. In this study, we developed in vitro models of the KRAS G12C cancer, derived from resistant clones against sotorasib and adagrasib, and searched for secondary KRAS mutations as on-target resistance mechanisms to develop possible strategies to overcome such resistance.

METHODS

We chronically exposed Ba/F3 cells transduced with KRAS^G12C to sotorasib or adagrasib in the presence of N-ethyl-N-nitrosourea and searched for secondary KRAS mutations. Strategies to overcome resistance were also investigated.

RESULTS

We generated 142 Ba/F3 clones resistant to either sotorasib or adagrasib, of which 124 (87%) harbored secondary KRAS mutations. There were 12 different secondary KRAS mutations. Y96D and Y96S were resistant to both inhibitors. A combination of novel SOS1 inhibitor, BI-3406, and trametinib had potent activity against this resistance. Although G13D, R68M, A59S and A59T, which were highly resistant to sotorasib, remained sensitive to adagrasib, Q99L was resistant to adagrasib but sensitive to sotorasib.

CONCLUSIONS

We identified many secondary KRAS mutations causing resistance to sotorasib, adagrasib, or both, in vitro. The differential activities of these two inhibitors depending on the secondary mutations suggest sequential use in some cases. In addition, switching to BI-3406 plus trametinib might be a useful strategy to overcome acquired resistance owing to the secondary Y96D and Y96S mutations.

One Atom Makes All the Difference: Getting a Foot in the Door between SOS1 and KRAS

KRAS, the most common oncogenic driver in human cancers, is controlled and signals primarily through protein-protein interactions (PPIs). The interaction between KRAS and SOS1, crucial for the activation of KRAS, is a typical, challenging PPI with a large contact surface area and high affinity. Here, we report that the addition of only one atom placed between Y884^SOS1 and A73^KRAS is sufficient to convert SOS1 activators into SOS1 inhibitors. We also disclose the discovery of BI-3406. Combination with the upstream EGFR inhibitor afatinib shows in vivo efficacy against KRAS^G13D mutant colorectal tumor cells, demonstrating the utility of BI-3406 to probe SOS1 biology. These findings challenge the dogma that large molecules are required to disrupt challenging PPIs. Instead, a "foot in the door" approach, whereby single atoms or small functional groups placed between key PPI interactions, can lead to potent inhibitors even for challenging PPIs such as SOS1-KRAS.

BI-3406, a Potent and Selective SOS1-KRAS Interaction Inhibitor, Is Effective in KRAS-Driven Cancers through Combined MEK Inhibition

KRAS is the most frequently mutated driver of pancreatic, colorectal, and non-small cell lung cancers. Direct KRAS blockade has proved challenging, and inhibition of a key downstream effector pathway, the RAF-MEK-ERK cascade, has shown limited success because of activation of feedback networks that keep the pathway in check. We hypothesized that inhibiting SOS1, a KRAS activator and important feedback node, represents an effective approach to treat KRAS-driven cancers. We report the discovery of a highly potent, selective, and orally bioavailable small-molecule SOS1 inhibitor, BI-3406, that binds to the catalytic domain of SOS1, thereby preventing the interaction with KRAS. BI-3406 reduces formation of GTP-loaded RAS and limits cellular proliferation of a broad range of KRAS-driven cancers. Importantly, BI-3406 attenuates feedback reactivation induced by MEK inhibitors and thereby enhances sensitivity of KRAS-dependent cancers to MEK inhibition. Combined SOS1 and MEK inhibition represents a novel and effective therapeutic concept to address KRAS-driven tumors. SIGNIFICANCE: To date, there are no effective targeted pan-KRAS therapies. In-depth characterization of BI-3406 activity and identification of MEK inhibitors as effective combination partners provide an attractive therapeutic concept for the majority of KRAS-mutant cancers, including those fueled by the most prevalent mutant KRAS oncoproteins, G12D, G12V, G12C, and G13D.See related commentary by Zhao et al., p. 17.This article is highlighted in the In This Issue feature, p. 1.

Antitumor Activity of the IGF-1/IGF-2-Neutralizing Antibody Xentuzumab (BI 836845) in Combination with Enzalutamide in Prostate Cancer Models

Androgen deprivation therapy and second-generation androgen receptor signaling inhibitors such as enzalutamide are standard treatments for advanced/metastatic prostate cancer. Unfortunately, most men develop resistance and relapse; signaling via insulin-like growth factor (IGF) has been implicated in castration-resistant prostate cancer. We evaluated the antitumor activity of xentuzumab (IGF ligand-neutralizing antibody), alone and in combination with enzalutamide, in prostate cancer cell lines (VCaP, DuCaP, MDA PCa 2b, LNCaP, and PC-3) using established in vitro assays, and in vivo, using LuCaP 96CR, a prostate cancer patient-derived xenograft (PDX) model. Xentuzumab + enzalutamide reduced the viability of phosphatase and tensin homolog (PTEN)-expressing VCaP, DuCaP, and MDA PCa 2b cells more than either single agent, and increased antiproliferative activity and apoptosis induction in VCaP. Xentuzumab or xentuzumab + enzalutamide inhibited IGF type 1 receptor and AKT serine/threonine kinase (AKT) phosphorylation in VCaP, DuCaP, and MDA PCa 2b cells; xentuzumab had no effect on AKT phosphorylation and proliferation in PTEN-null LNCaP or PC-3 cells. Knockdown of PTEN led to loss of antiproliferative activity of xentuzumab and reduced activity of xentuzumab + enzalutamide in VCaP cells. Xentuzumab + enzalutamide inhibited the growth of castration-resistant LuCaP 96CR PDX with acquired resistance to enzalutamide, and improved survival in vivo The data suggest that xentuzumab + enzalutamide combination therapy may overcome castration resistance and could be effective in patients who are resistant to enzalutamide alone. PTEN status as a biomarker of responsiveness to combination therapy needs further investigation.

Selective and Potent CDK8/19 Inhibitors Enhance NK-Cell Activity and Promote Tumor Surveillance

Natural killer (NK) cells play a pivotal role in controlling cancer. Multiple extracellular receptors and internal signaling nodes tightly regulate NK activation. Cyclin-dependent kinases of the mediator complex (CDK8 and CDK19) were described as a signaling intermediates in NK cells. Here, we report for the first time the development and use of CDK8/19 inhibitors to suppress phosphorylation of STAT1^S727 in NK cells and to augment the production of the cytolytic molecules perforin and granzyme B (GZMB). Functionally, this resulted in enhanced NK-cell-mediated lysis of primary leukemia cells. Treatment with the CDK8/19 inhibitor BI-1347 increased the response rate and survival of mice bearing melanoma and breast cancer xenografts. In addition, CDK8/19 inhibition augmented the antitumoral activity of anti-PD-1 antibody and SMAC mimetic therapy, both agents that promote T-cell-mediated antitumor immunity. Treatment with the SMAC mimetic compound BI-8382 resulted in an increased number of NK cells infiltrating EMT6 tumors. Combination of the CDK8/19 inhibitor BI-1347, which augments the amount of degranulation enzymes, with the SMAC mimetic BI-8382 resulted in increased survival of mice carrying the EMT6 breast cancer model. The observed survival benefit was dependent on an intermittent treatment schedule of BI-1347, suggesting the importance of circumventing a hyporesponsive state of NK cells. These results suggest that CDK8/19 inhibitors can be combined with modulators of the adaptive immune system to inhibit the growth of solid tumors, independent of their activity on cancer cells, but rather through promoting NK-cell function.

Abstract A030: Evaluation of phosphoprotein- and transcript-based pharmacodynamic biomarkers in pre-clinical studies of the novel SOS1::KRAS inhibitor BI-3406

KRAS is among the most frequently mutated oncogenes across all human cancers. Targeting KRAS-mutant cancers either directly or at the effector level has so far failed in the clinic, with adoptive responses and toxicities being the major limiting factors. Nevertheless, direct targeting of the mutant KRAS-G12C allele has shown promising results in early clinical trials. BI-3406 is an inhibitor of the interaction between KRAS and its Guanine Nucleotide Exchange Factor (GEF) SOS1. The efficient loading of both mutant as well as wild-type KRAS with GTP depends on SOS1. We present the activity of the potent and selective SOS1::KRAS inhibitor BI-3406 in monotherapy and combinations in a different presentation at this meeting (Marco H Hofmann et al.). In this work, we focus on the characterization of suitable pharmacodynamic biomarkers in pre-clinical studies. Analysis of the well-studied RAS-MAPK effector p-ERK demonstrates BI-3406 treatment induced modulation in the majority of tested cell lines, independent of their KRAS mutation status. Interestingly, the effects on p-AKT levels, linked to PI3K signaling, were much less pronounced, indicating that the MAPK pathway is the major effector pathway of mutant KRAS singnaling. Confirmatory results were obtained from a technically independent and orthogonal platform. These findings are supported by a genome-wide transcriptome analysis, which revealed well established feedback regulators and p-ERK targets such as DUSP6, EGR1, or ETV5 as robust pharmacodynamic biomarkers in response to SOS1::KRAS inhibitor treatment. Importantly, we also compare biomarker responses of SOS1::KRAS inhibition to direct KRAS-G12C inhibition. In summary, these transcript-based biomarkers add value over conventional downstream signaling protein biomarkers such as p-ERK and will be implemented in future clinical trials.

Citation Format: Daniel Gerlach, Fabio Savarese, Marco H Hofmann, Francesca Trapani, Michael Gmachl, Dana-Adriana Botesteanu, Jürgen Ramharter, Heribert Arnhof, Norbert Schweifer, Darryl B McConnell, Norbert Kraut. Evaluation of phosphoprotein- and transcript-based pharmacodynamic biomarkers in pre-clinical studies of the novel SOS1::KRAS inhibitor BI-3406 [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A030. doi:10.1158/1535-7163.TARG-19-A030

Abstract PL06-01: Discovery of BI-3406: A potent and selective SOS1::KRAS inhibitor opens a new approach for treating KRAS-driven tumors

KRAS is the most frequently mutated oncogene with high prevalence in pancreatic, colorectal, and non-small cell lung tumors. KRAS signaling is tightly regulated and various factors, including negative feedback pathways have limited the clinical efficacy of inhibitors of downstream MAPK signaling in the KRAS mutant context. Here we report the discovery of BI-3406 and demonstrate it is a highly potent and selective, orally bioavailable SOS1::KRAS inhibitor which binds to the catalytic domain of the guanine nucleotide exchange factor (GEF) SOS1 thereby preventing the interaction with KRAS-GDP. BI-3406 does not block the interaction of KRAS with SOS2 but elicits activity on a broad panel of KRAS oncogenic variants, including all major G12 and G13 oncoproteins. In KRAS-dependent cancers, BI-3406 potently reduces the formation of GTP-loaded KRAS, and inhibits MAPK pathway signaling. Down-modulation of this signaling cascade by BI-3406 in KRAS G12 or G13 mutant cells effectively limits cell proliferation. As a monotherapy, BI-3406 modulates signaling, as assessed by p-ERK and target genes, and displays marked anti-tumor efficacy in KRAS mutant xenografts. Due to BI-3406 blocking the negative feedback relief induced by MAPK inhibition, it has the potential to sensitize KRAS-dependent cancers to MEK inhibitor treatment. Combination with MEK inhibition leads to profound pathway blockade and tumor regressions in vivo. The combination of SOS1 and MEK inhibition is a potential therapy for the majority of KRAS-driven cancers including those fuelled by the most prevalent KRAS mutant oncoproteins. Furthermore, the pharmacological properties of BI-3406 and close analogues hold the promise of a significant treatment benefit in a broad patient population that is currently lacking precision medicine options. A Phase 1 clinical trial is in preparation for patients with advanced KRAS-mutated cancers to evaluate safety, tolerability, pharmacokinetic and pharmacodynamic properties, and preliminary efficacy of BI 1701963, a SOS1::KRAS inhibitor closely related to BI-3406.

Citation Format: Marco H Hofmann, Michael Gmachl, Jürgen Ramharter, Fabio Savarese, Daniel Gerlach, Joseph R Marszalek, Michael P Sanderson, Francesca Trapani, Dirk Kessler, Klaus Rumpel, Dana-Adriana Botesteanu, Peter Ettmayer, Heribert Arnhof, Thomas Gerstberger, Christiane Kofink, Tobias Wunberg, Szu-Chin Fu, Jessica Teh, Christopher P. Vellano, Jonathan C. O’Connell, Rachel L Mendes, Juergen Moll, Timothy P. Heffernan, Mark Pearson, Darryl B McConnell, Norbert Kraut. Discovery of BI-3406: A potent and selective SOS1::KRAS inhibitor opens a new approach for treating KRAS-driven tumors [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr PL06-01. doi:10.1158/1535-7163.TARG-19-PL06-01

The novel BET bromodomain inhibitor BI 894999 represses super-enhancer-associated transcription and synergizes with CDK9 inhibition in AML

Bromodomain and extra-terminal (BET) protein inhibitors have been reported as treatment options for acute myeloid leukemia (AML) in preclinical models and are currently being evaluated in clinical trials. This work presents a novel potent and selective BET inhibitor (BI 894999), which has recently entered clinical trials (NCT02516553). In preclinical studies, this compound is highly active in AML cell lines, primary patient samples, and xenografts. HEXIM1 is described as an excellent pharmacodynamic biomarker for target engagement in tumors as well as in blood. Mechanistic studies show that BI 894999 targets super-enhancer-regulated oncogenes and other lineage-specific factors, which are involved in the maintenance of the disease state. BI 894999 is active as monotherapy in AML xenografts, and in addition leads to strongly enhanced antitumor effects in combination with CDK9 inhibitors. This treatment combination results in a marked decrease of global p-Ser2 RNA polymerase II levels and leads to rapid induction of apoptosis in vitro and in vivo. Together, these data provide a strong rationale for the clinical evaluation of BI 894999 in AML.

The IGF1R/INSR Inhibitor BI 885578 Selectively Inhibits Growth of IGF2-Overexpressing Colorectal Cancer Tumors and Potentiates the Efficacy of Anti-VEGF Therapy

Clinical studies of pharmacologic agents targeting the insulin-like growth factor (IGF) pathway in unselected cancer patients have so far demonstrated modest efficacy outcomes, with objective responses being rare. As such, the identification of selection biomarkers for enrichment of potential responders represents a high priority for future trials of these agents. Several reports have described high IGF2 expression in a subset of colorectal cancers, with focal IGF2 amplification being responsible for some of these cases. We defined a novel cut-off value for IGF2 overexpression based on differential expression between colorectal tumors and normal tissue samples. Analysis of two independent colorectal cancer datasets revealed IGF2 to be overexpressed at a frequency of 13% to 22%. An in vitro screen of 34 colorectal cancer cell lines revealed IGF2 expression to significantly correlate with sensitivity to the IGF1R/INSR inhibitor BI 885578. Furthermore, autocrine IGF2 constitutively activated IGF1R and Akt phosphorylation, which was inhibited by BI 885578 treatment. BI 885578 significantly delayed the growth of IGF2-high colorectal cancer xenograft tumors in mice, while combination with a VEGF-A antibody increased efficacy and induced tumor regression. Besides colorectal cancer, IGF2 overexpression was detected in more than 10% of bladder carcinoma, hepatocellular carcinoma and non-small cell lung cancer patient samples. Meanwhile, IGF2-high non-colorectal cancer cells lines displayed constitutive IGF1R phosphorylation and were sensitive to BI 885578. Our findings suggest that IGF2 may represent an attractive patient selection biomarker for IGF pathway inhibitors and that combination with VEGF-targeting agents may further improve clinical outcomes. Mol Cancer Ther; 16(10); 2223-33. ©2017 AACR.

Abstract 4630: Development of selective and potent CDK8 inhibitors that increase NK cell activity, which translates in tumor surveillance

Background: Cyclin-dependent kinase 8 (CDK8) is part of the mediator complex that can either positively or negatively influence transcription. CDK8 is known to phosphorylate signal transducer and activator of transcription 1 (STAT1) at the position Ser727. STAT1 activity is regulated by JAK-mediated phosphorylation of tyrosine701 which leads to dimerization, nuclear translocation and IFN-γ induced phosphorylation mediated by CDK8. Introduction of an alanine mutation at the phosphorylation site STAT1-S727 results in enhanced NK cell cytotoxicity accompanied by increased levels of perforin and granzyme B (Putz et al. 2013).

Method: Here we present the discovery and development of potent and selective CDK8 inhibitors guided by crystallography. The inhibitory effect of optimized compounds BI 9811 and BI 1347 on STAT1 phosphorylation and perforin release was investigated in the human NK cell line NK-92MI. Direct effects on cancer cells were furthermore analyzed in a broad panel of cell lines. The compound BI 1347 was profiled in vivo in the orthotopic B16-F10 melanoma mouse model.

Results: Highly potent and selective CDK8 inhibitors were identified with an IC50 of below 10 nM in a biochemical kinase assay, which translated in a potent down regulation of the STAT1- Ser727 signal and in increased perforin and granzyme B secretion. BI 9811 and BI 1347 were highly selective for CDK8, as tested in a broad kinase panel and showed no cytotoxic activity on NK cells and most cancer cell lines, which distinguishes this compound class from published CDK8 inhibitors. A representative molecule out of this compound class demonstrated in vivo biomarker modulation and survival increase in the murine B16-F10 melanoma mouse model.

Conclusion: We developed potent CDK8 inhibitors that show activation of NK cells that translates into biomarker modulation (pSTAT1Ser727) and in vivo efficacy.

Citation Format: Marco H. Hofmann, Harald Engelhardt, Sebastian Carotta, Heribert Arnhof, Dirk Scharn, Marc Kerenyi, Moritz Mayer, Gerhard Gmaschitz, Georg Egger, Christian Engelhardt, Michael Sanderson, Maria A. Impagnatiello, Renate Schnitzer, Mark Pearson, Darryl McConnell, Norbert Kraut, Jürgen Moll. Development of selective and potent CDK8 inhibitors that increase NK cell activity, which translates in tumor surveillance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4630. doi:10.1158/1538-7445.AM2017-4630

Abstract 20: Xentuzumab, a humanized IGF-1 and IGF-2 ligand neutralizing antibody, improves the antitumor efficacy of enzalutamide in preclinical models of prostate cancer

Background: The proliferative and pro-survival signals driven by the insulin-like growth factor (IGF) ligands, IGF-1 and IGF-2, are transmitted through their binding to the IGF-1 receptor (IGF-1R). In addition, IGF-2 activates the insulin receptor variant A (IR-A) that is expressed during embryonic development as well as in many cancers. A large body of preclinical evidence suggests that IGF signaling plays a key role in cancer by driving therapy resistance, due to cross-talk with other signaling networks such as androgen receptor signaling. The aim of this study was to explore the potential of the IGF-1/-2 ligand blocking antibody, xentuzumab (BI 836845[1]), to enhance the anti-tumor activity of enzalutamide in prostate cancer cell lines and in a patient-derived prostate cancer xenograft model.

Methods: Effects of enzalutamide, xentuzumab and combinations thereof on in vitro proliferation, survival, cell cycle and signaling were evaluated using the prostate cancer cell lines VCaP, DuCaP, MDA PCa 2b, and LNCaP. The in vivo efficacy of enzalutamide, alone and in combination with xentuzumab was investigated using LuCaP 96CR, a patient-derived xenograft model of castration-resistant prostate cancer. Tumors were implanted s.c. into castrate SCID mice. When tumors exceeded 150mm3 animals were randomized into groups: 1) Control; 2) enzalutamide (50 mg/kg QD po), 3) xentuzumab (BI 836845[1], 200 mg/kg QW ip) in combination with enzalutamide.

Results: Cell viability was more effectively reduced by the combination of enzalutamide and xentuzumab than either drug alone in three of four cell lines expressing the IGF-1R and the androgen receptor (AR). In VCaP cells, prolonged inhibition of IGF pathway signaling and enhanced blockade of proliferation as well as induction of apoptosis was observed after combination treatment. In vivo, enzalutamide monotherapy did not show significant antitumor efficacy in the LuCaP 96CR model, however, combined treatment with xentuzumab significantly inhibited progression of LuCaP 96CR tumor growth (p<0.001 vs. enzalutamide alone). Reduced serum PSA levels were observed after enzalutamide and combination treatment. Enzalutamide plus xentuzumab inhibited tumor growth at tolerated doses and resulted in significant improvements in survival.

Conclusions: These studies demonstrated that addition of the IGF-1/-2 neutralizing antibody xentuzumab to enzalutamide results in improved anti-neoplastic activity in a subset of prostate cancer cell lines in vitro, and to re-sensitization to enzalutamide in a patient-derived xenograft model of CRPC.

Reference:

[1] Friedbichler K et al. (2014). Mol Cancer Ther 13(2):399-409.

Citation Format: Ulrike Weyer-Czernilofsky, Marco H. Hofmann, Paul J. Adam, Flavio Solca, Katrin Friedbichler, Norbert Kraut, Eva Corey, Thomas Bogenrieder. Xentuzumab, a humanized IGF-1 and IGF-2 ligand neutralizing antibody, improves the antitumor efficacy of enzalutamide in preclinical models of prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 20. doi:10.1158/1538-7445.AM2017-20

Structure-Based Design of an in Vivo Active Selective BRD9 Inhibitor

Components of the chromatin remodelling switch/sucrose nonfermentable (SWI/SNF) complex are recurrently mutated in tumors, suggesting that altering the activity of the complex plays a role in oncogenesis. However, the role that the individual subunits play in this process is not clear. We set out to develop an inhibitor compound targeting the bromodomain of BRD9 in order to evaluate its function within the SWI/SNF complex. Here, we present the discovery and development of a potent and selective BRD9 bromodomain inhibitor series based on a new pyridinone-like scaffold. Crystallographic information on the inhibitors bound to BRD9 guided their development with respect to potency for BRD9 and selectivity against BRD4. These compounds modulate BRD9 bromodomain cellular function and display antitumor activity in an AML xenograft model. Two chemical probes, BI-7273 (1) and BI-9564 (2), were identified that should prove to be useful in further exploring BRD9 bromodomain biology in both in vitro and in vivo settings.

Efficacy and mechanism of action of volasertib, a potent and selective inhibitor of Polo-like kinases, in preclinical models of acute myeloid leukemia

Polo-like kinase 1 (Plk1), a member of the Polo-like kinase family of serine/threonine kinases, is a key regulator of multiple steps in mitosis. Here we report on the pharmacological profile of volasertib, a potent and selective Plk inhibitor, in multiple preclinical models of acute myeloid leukemia (AML) including established cell lines, bone marrow samples from AML patients in short-term culture, and subcutaneous as well as disseminated in vivo models in immune-deficient mice. Our results indicate that volasertib is highly efficacious as a single agent and in combination with established and emerging AML drugs, including the antimetabolite cytarabine, hypomethylating agents (decitabine, azacitidine), and quizartinib, a signal transduction inhibitor targeting FLT3. Collectively, these preclinical data support the use of volasertib as a new therapeutic approach for the treatment of AML patients, and provide a foundation for combination approaches that may further improve and prolong clinical responses.

Discovery of novel amino-pyrimidine inhibitors of the insulin-like growth factor 1 (IGF1R) and insulin receptor (INSR) kinases; parallel optimization of cell potency and hERG inhibition

Modulating hERG activity of IGF1R/INSR inhibitors by tuning log D and use of homology modelling.

Pharmacodynamic and antineoplastic activity of BI 836845, a fully human IGF ligand-neutralizing antibody, and mechanistic rationale for combination with rapamycin

Insulin-like growth factor (IGF) signaling is thought to play a role in the development and progression of multiple cancer types. To date, therapeutic strategies aimed at disrupting IGF signaling have largely focused on antibodies that target the IGF-I receptor (IGF-IR). Here, we describe the pharmacologic profile of BI 836845, a fully human monoclonal antibody that utilizes an alternative approach to IGF signaling inhibition by selectively neutralizing the bioactivity of IGF ligands. Biochemical analyses of BI 836845 demonstrated high affinity to human IGF-I and IGF-II, resulting in effective inhibition of IGF-induced activation of both IGF-IR and IR-A in vitro. Cross-reactivity to rodent IGFs has enabled rigorous assessment of the pharmacologic activity of BI 836845 in preclinical models. Pharmacodynamic studies in rats showed potent reduction of serum IGF bioactivity in the absence of metabolic adverse effects, leading to growth inhibition as evidenced by reduced body weight gain and tail length. Moreover, BI 836845 reduced the proliferation of human cell lines derived from different cancer types and enhanced the antitumor efficacy of rapamycin by blocking a rapamycin-induced increase in upstream signaling in vitro as well as in human tumor xenograft models in nude mice. Our data suggest that BI 836845 represents a potentially more effective and tolerable approach to the inhibition of IGF signaling compared with agents that target the IGF-I receptor directly, with potential for rational combinations with other targeted agents in clinical studies.

BI 836845, a fully human IGF ligand neutralizing antibody, to improve the efficacy of rapamycin by blocking rapamycin-induced AKT activation

3092

Background: Analogs of rapamycin (rapalogs) targeting mammalian target of rapamycin complex 1 (mTORC1) have shown clinical activity in several cancers. Nonetheless, preclinical and clinical data suggest that there may be intrinsic resistance to rapalogs through a feedback loop which activates upstream signaling when mTORC1 is blocked. BI 836845 is a fully human antibody, currently in phase I clinical trials, which potently neutralizes both IGF-1 and IGF-2. We tested whether BI 836845 is able to improve the efficacy of rapamycin by inhibiting upstream signaling in preclinical models. Methods: Cancer cell lines were profiled in vitro and in vivo for sensitivity to BI 836845 and rapamycin, alone or in combination. Mitogenic signaling was examined by measuring levels of phosphorylated AKT (pAKT) using Western blot analysis. IGF bioactivity was determined using a cellular IGF-1R phosphorylation ELISA. Results: The combination of BI 836845 and rapamycin was more effective than either agent alone at inhibiting the proliferation of Ewing’s sarcoma cells cultured in vitro as well as in a nude mouse xenograft model in vivo. Analysis of cell signaling upstream of mTOR demonstrated that treatment with rapamycin alone resulted in elevated pAKT, indicating feedback loop activation. BI 836845 treatment alone or in combination with rapamycin inhibited AKT phosphorylation, demonstrating that the rapamycin-induced increase in pAKT was due to elevated IGF bioactivity. Consistent with this we demonstrated that rapamycin increased IGF bioactivity in mice and that this could be inhibited by BI 836845. We extended these studies to include other cancer cell lines and profiled the correlation between improved efficacy of the combination with BI 836845 inhibition of rapamycin-induced feedback. A correlation has been observed for cancer cells derived from several indications. Conclusions: Rapamycin treatment increases AKT activation via elevated IGF ligand bioactivity. This effect can be inhibited by BI 836845, thus explaining the improved pre-clinical efficacy seen when both agents are combined. These data provide a rationale for the clinical combination of rapalogs and BI 836845.

Abstract LB-397: Functional characterization of IGF-1R antibodies and possible implications for clinical safety and efficacy

Background

This is the first study performing a head to head comparison of monoclonal IGF-1R antibodies (mAb) based on published sequences for therapeutic mAbs from Pfizer (CP751,871, IgG2 kappa), Amgen (AMG479, IgG1 lambda), Merck (h7C10, cloned to R1507 backbone), Imclone (IMC-A12, IgG1 lambda) and Roche (R1507, IgG1).

Methods

In this study, sequence information for IGF-1R mAbs was extracted from patents and used to clone and transiently express IgGs (*=re-synthesized) in HEK293F cells. In vitro assays for ligand binding, IGF-1R auto-phosphorylation, IGF-1R downregulation, IR co-downregulation, and affinity analyses (Biacore) were used. In vivo comparison was done in BxPC3 xenograft mouse model.

Results

All antibodies inhibit IGF-1 binding and signaling at low nanomolar levels. While IMC-A12* and R1507 also prevent IGF-2 binding to IGF-1R and subsequent receptor activation, CP751,871* and h7C10* do not inhibit IGF-2 binding and have only limited impact (55%/30% inhibition) on IGF-2 signaling.

Analysis of IGF-1R phosphorylation in 0.5 % FCS medium revealed that all antibodies except R1507 exert agonistic activity. Interestingly, Fab fragments of agonistic mAbs became antagonistic, indicating that bivalent binding is necessary for agonistic effects.

All mAb (200nM, 24h treatment of MCF-7 cells) with the exception of AMG479* efficiently downregulated 78–82% the IGF-1R. Analysis of the same cell lysates revealed however striking differences in IR co-downregulation, a mechanism discussed as possible cause of clinical hyperglycemia. R1507 had the least side effects on Insulin co-downregulation (9%) compared to h7C10* (15%), CP751,871* (23%) and IMC-A12* (46%).

Differences were also seen in the binding kinetics. Both AMG479* and R1507 showed faster koff rates resulting in shorter retention times at the receptor. Since kon/koff rates are discussed to influence tumor penetration (1), we compared downregulation of IGF-1R by R1507 and CP751,871* in xenograft tumors. Although both mAbs downregulate IGF-1R in vitro to the same extent, R1507 was significantly more effective in the in-vivo setting.

Conclusion

The head to head comparison of IGF-1R mAbs revealed differences in regard to binding properties, tumor penetration, IR codownregulation, and inhibition of signaling via the ligand IGF-2.

Reference List

1. Adams, G. P., Schier, R., McCall, A. M., Simmons, H. H., Horak, E. M., Alpaugh, R. K., Marks, J. D., and Weiner, L. M. (2001) Cancer Res. 61, 4750–4755

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-397. doi:10.1158/1538-7445.AM2011-LB-397

Abstract LB-212: XGFR, an Fc-engineered dual signaling inhibitor targeting IGF-1R and EGFR

Background: Elevated signaling via the receptor tyrosine kinases IGF-1R and EGFR has been identified as common characteristic of multiple cancer type. IGF-1R and EGFR signal predominantly through the PI3K and MAPK signaling pathways and thereby mediate growth and survival signals crucial for the development and progression of cancer. There is strong cross talk on multiple levels between IGF-1R and EGFR dependent signaling pathways. Therefore, targeting IGF-1R and EGFR simultaneously is an attractive way to achieve maximal inhibition of signal transduction and to avoid resistance formation.

Methods: Bispecific IGF1R-EGFR antibodies were engineered by linking scFv domains of an EGFR Mab (GA201) via Serine-Glycine linkers to an IgG1 IGF-1R Mab (RG1507). The functional properties of the bispecific antibodies were evaluated in cellular in vitro assays (IGF-1R/EGFR phosphorylation, downregulation, 3D proliferation and ADCC assays) and in in vivo xenograft models for tumor growth inhibition and survival.

Results: Bispecific IGF-1R-EGFR antibodies (XGFR2, XGFR3, XGFR4) were successfully generated with yields and stability comparable to conventional IgG1 antibodies. XGFR antibodies effectively inhibited IGF-1R and EGFR phosphorylation and 3D proliferation in H322M tumor cells and induced strong downmodulation of IGF-1R and enhanced EGFR downmodulation compared to the parental EGFR antibody GA201. XGFR antibodies showed strong anti-tumor efficacy comparable to the combination of monospecific IGF-1R and EGFR Mabs in the BxPC3 and H322M xenograft models. To enhance the ADCC properties of XGFR, afucosylated, glycoengineered bispecific antibodies with enhanced affinity for FcγRIIIA were generated using the GlycoMab technology. Glycoengineered bispecific antibodies were shown to have superior ADCC properties in in vitro ADCC assays and XGFR4 significantly prolonged median and overall survival of mice in an ADCC competent in vivo model (A549 i.v.).

Conclusions: Bispecific IGF-1R-EGFR antibodies represent an attractive therapeutic strategy to simultaneously target two key components of multiple cancer types (IGF-1R and EGFR), resulting in effective inhibition of the PI3K and MAPK signaling pathway and to avoid the formation of resistance to therapy. Having overcome issues of stability and productivity, bispecific antibodies may become an advantageous way to reduce costs and infusion times in cancer therapy, while at the same time, achieving maximal anti-tumor effects through inhibition of multiple targets.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-212. doi:10.1158/1538-7445.AM2011-LB-212

A short hairpin DNA analogous to miR-125b inhibits C-Raf expression, proliferation, and survival of breast cancer cells

The noncoding RNA miR-125b has been described to reduce ErbB2 protein expression as well as proliferation and migration of cancer cell lines. As additional target of miR-125b, we identified the c-raf-1 mRNA by sequence analysis. We designed a short hairpin-looped oligodeoxynucleotide (ODN) targeted to the same 3' untranslated region of c-raf-1 mRNA as miR-125b. The fully complementary ODN antisense strand is linked to a second strand constituting a partially double-stranded structure of the ODN. Transfection of the c-raf-1-specific ODN (ODN-Raf) in a breast cancer cell line reduced the protein levels of C-Raf, ErbB2, and their downstream effector cyclin D1 similar to miR-125b. MiR-125b as well as ODN-Raf showed no effect on the c-raf-1 mRNA level in contrast to small interfering RNA. Unlike miR-125b, ODN-Raf induced a cytopathic effect. This may be explained by the structural properties of ODN-Raf, which can form G-tetrads. Thus, the short hairpin-looped ODN-Raf, targeting the same region of c-raf-1 as miR-125b, is a multifunctional molecule reducing the expression of oncoproteins and stimulating cell death. Both features may be useful to interfere with tumor growth.

Receptive field organization of electrosensory neurons in the paddlefish (Polyodon spathula)

Paddlefish use their electrosense to locate small water fleas (daphnia), their primary prey, in three-dimensional space. High sensitivity and a representation of object location are essential for this task. High sensitivity can be achieved by convergence of information from a large number of receptors and object location is usually represented in the nervous system by topographic maps. However the first electrosensory center in the brain, the dorsal octavolateral nucleus in the hindbrain, is neither topographically organized nor does it show a higher sensitivity than primary afferent fibers. Here, we investigated the response properties of electrosensory neurons in the dorsal octavolateral nucleus (DON), the lateral mesencephalic nucleus (LMN) and the tectum mesencephali (TM). LMN units are characterized by large receptive fields, which suggest a high degree of convergence. TM units have small receptive fields and are topographically arranged, at least in the rostro-caudal axis, the only dimension we could test. Well-defined receptive fields, however, could only be detected in the TM with a moving DC stimulus. The receptive fields of TM units, as determined by slowly scanning the rostrum and head with a 5 Hz stimulus, were very large and frequently two or more receptive fields were present. The receptive fields for LMN units were located in the anterior half of the rostrum whereas TM units had receptive fields predominantly on the head and at the base of the rostrum. A detailed analysis of the prey catching behavior revealed that it consists of two phases that coincide with the location of the receptive fields in LMN and TM, respectively. This suggests that LMN units are responsible for the initial orienting response that occurs when the prey is alongside the anterior first half of the rostrum. TM units, in contrast, had receptive fields at locations where the prey is located when the fish opens its mouth and attempts the final strike.

Response properties of electrosensory units in the midbrain tectum of the paddlefish (Polyodon spathula Walbaum)

Paddlefish use their peculiar rostrum to detect minute electric fields from their main prey, small water fleas. Electroreceptors over the rostrum and head sense these fields and send the information into a single hindbrain area, the dorsal octavolateral nucleus (DON). From there, information is sent to various midbrain structures, including the tectum. The response properties of primary afferent fibers and DON units has been well investigated, but nothing is known about electrosensory units in the midbrain. Here we recorded the responses of single units in the midbrain tectum and DON to uniform electric fields. Tectal units exhibited little spontaneous activity and responded to sine waves with a few, well phase-locked spikes. Phase locking was still significant at amplitudes one order of magnitude lower than in the DON. If stimulated with sinusoidal electric fields of different frequencies, phase locking in DON units decreased proportionally with frequency whereas the response of tectal units depended little on frequency. This is in agreement with behavioral studies showing that relevant frequencies range from DC to ca 20 Hz.

Polymorphic CYP2B6: molecular mechanisms and emerging clinical significance

Polymorphisms in drug-metabolizing enzymes and drug transporters contribute to wide and inheritable variability in drug pharmacokinetics, response and toxicity. One of the less well-studied human cytochrome P450s is (CYP)2B6, a homologue of the rodent phenobarbital-inducible CYP2B enzymes. Clinically used drug substrates include cytostatics (cyclophosphamide), HIV drugs (efavirenz and nevirapine), antidepressants (bupropion), antimalarials (artemisinin), anesthetics (propofol) and synthetic opioids (methadone). Contrary to the model polymorphisms of CYP2D6 and CYP2C19, which were discovered by adverse drug reactions, pharmacogenetic study of CYP2B6 was initiated by reverse genetics approaches and subsequent functional and clinical studies. With over 100 described SNPs, numerous complex haplotypes and distinct ethnic frequencies, CYP2B6 is one of the most polymorphic CYP genes in humans. In this review, we summarize general biomolecular and pharmacological features and present a detailed up-to-date description of genetic polymorphisms, including a discussion of recent clinical applications of CYP2B6 pharmacogenetics.

Aberrant splicing caused by single nucleotide polymorphism c.516G>T [Q172H], a marker of CYP2B6*6, is responsible for decreased expression and activity of CYP2B6 in liver

CYP2B6 is a polymorphic human drug metabolizing cytochrome P450 with clinical relevance for several drug substrates including cyclophosphamide, bupropion, and efavirenz. The common allele CYP2B6*6 [c. 516G>T, Q172H, and c.785A>G, K262R] has previously been associated with lower expression in human liver and with increased plasma levels of efavirenz in human immunodeficiency virus patients, but the molecular mechanism has remained unclear. We present novel data showing that hepatic CYP2B6 mRNA levels are reduced in *6 carriers, suggesting a pretranslational mechanism resulting in decreased expression. As one possibility, we first analyzed the common promoter variant, -750T>C, but the results did not suggest a prominent role in phenotype determination. In contrast, analysis of liver mRNA splicing variants demonstrated that the most common form lacking exons 4 to 6 (SV1) was tightly associated with the *6 allele and apparently also with the rare variant c.777C>A(CYP2B6*3). Further investigation using minigene constructs transfected into eukaryotic cell lines COS-1 and Huh7 demonstrated that the single nucleotide polymorphism c.516G>T in allele CYP2B6*6 was alone responsible for aberrant splicing resulting in high-splice variant (SV) 1 and low-CYP2B6 expression phenotype. Minigenes carrying the single c.785A>G polymorphism or the rare c.777C>A variant resulted in normal and intermediate expression phenotypes, respectively. In conclusion, the mechanism of the common *6 allele involves predominantly aberrant splicing, thus leading to reduced functional mRNA, protein, and activity. These results establish the single nucleotide polymorphism 516G>Tasthe causal sequence variation for severely decreased expression and function associated with CYP2B6*6.

The ability of Rhodnius prolixus (Hemiptera; Reduviidae) to approach a thermal source solely by its infrared radiation

The ability of the haematophagous bug Rhodnius prolixus to approach a pure IR source only by its long-wave infrared radiation (IR) was investigated. To exclude any heated air from reaching the bug a cooled IR-transmitting window was placed between the IR-source (a Peltierelement heated to 35 degrees C) and the bug. Starved bugs were tested under invisible short-wave IR illumination (lambda<1 µm) in order to exclude also any visual cues. The number of bugs approaching the IR-source was significantly increased compared to the controls in which the IR-source was turned off (chi(2)-test, P<0.01). Our results show that Rhodnius prolixus can use infrared stimuli to find a host.

Effect of temperature and calcium on transneuronal diffusion of DiI in fixed brain preparations

The lipophilic tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was used to label neuronal pathways in fixed goldfish brains. The normal procedure involving 4% paraformaldehyde as a fixative, applying DiI and storing the brain in the fixative at 40 degrees C resulted in many cases in a rather diffuse labeling of fiber pathways and the occurrence of transneuronally labeled cells and fibers. We found that calcium and heat both facilitate the diffusion of DiI out of membranes in vibratome sections. We modified the protocol by adding the calcium binding substance ethylenediamine tetraacetate to all solutions and incubated at room temperature. This improved the sharpness of labeled structures and eliminated the transneuronal labeling in our material. Although transneuronal transport of the tracer may still occur under certain circumstances, the present modification of the DiI staining procedure substantially increased the staining quality and reproducibility and decreased the occurrence of transneuronal labeling.

Subdivisions of the olfactory system in the sterlet Acipenser ruthenus

Subdivisions of the olfactory system of the sterlet Acipenser ruthenus were investigated by means of horseradish peroxidase (HRP) injections into the nose, and by soybean agglutinin binding studies. With both methods primary olfactory fibers were labeled which projected to the ventral part of the glomerular layer of the olfactory bulb. The dorsal part of the olfactory bulb did not bind soybean agglutinin, however, even though HRP tracing showed primary olfactory fibers in that area. This confirms earlier morphological studies which claim the existence of distinct subdivisions of the olfactory system in the sturgeon. The lack of soybean agglutinin binding in the dorsal part of the olfactory bulb suggests, however, that this part is not homologous with the accessory olfactory system of tetrapods.

The extrabulbar olfactory pathway: primary olfactory fibers bypassing the olfactory bulb in bony fishes?

Recent evidence has revealed that some primary olfactory fibers bypass the olfactory bulb and terminate in tel- and/or diencephalic areas (extrabulbar olfactory pathway, EBOP). We investigated the projections of this system in different fishes by means of soybean agglutinin binding studies. In all species in which primary olfactory fibers were labelled, fiber bundles can be traced beyond the olfactory bulb. These run with the medial forebrain bundle and terminate at different targets, depending on the species. In the teleosts Macrognathus, Mogurnda, and Hemichromis, EBOP fibers can be traced into the ventral telencephalon, pars ventralis, pars supracommissuralis and/or into the preoptic nucleus. In most nonteleosts studied (Polypterus, Chalamoichthys, Amia), the EBOP also innervates diencephalic targets. An exception is Acipenser, which displays an innervation pattern similar to that in teleosts. Comparison with results obtained by other techniques suggests that the EBOP consists of primary olfactory fibers, which project not only to the olfactory bulb but also to various other targets in the prosencephalon of anamniotic vertebrates.

Retinopetal projections from diencephalic neurons in a primitive actinopterygian fish, the sterlet Acipenser ruthenus

Horseradish peroxidase (HRP) injections into the sterlet (Acipenser ruthenus) retina retrogradely label neurons in the dorso-medial thalamus, bilaterally. On the contralateral side, 5-7 cells were labelled, whereas ipsilaterally, only 2-3 cells were backfilled. Such diencephalic retinopetal cells have, so far, only been found in teleosts and in tetrapods. It has, therefore, been suggested that they evolved independently in these two vertebrate groups. Our findings on a primitive actinopterygian fish, suggest a more ancient origin of diencephalic projections to the retina.

Visual-evoked responses in the salmon telencephalon change during smolt transformation

During smolt transformation, salmon behaviour changes dramatically and the fish are imprinted to their natal stream. This brief episode is accompanied by changes in neurochemistry and connectivity of the brain. Comparing visual-evoked potentials, recorded from optic tectum and telencephalon by glass microcapillaries, we found pronounced differences before and after smolt transformation in the telencephalon. In the forebrain of presmolts, only small short-latency responses are present whereas postsmolts display large long-latency waves which characteristic dynamic properties in addition. These findings suggest changes in the functional role of the telencephalon during smolt transformation.

Interval-specific event related potentials to omitted stimuli in the electrosensory pathway in elasmobranchs: an elementary form of expectation

Multiunit activity and slow local field potentials show Omitted Stimulus Potentials (OSP) in the electrosensory system in rays (Platyrhinoidis triseriata, Urolophus halleri) after a missing stimulus in a 3 to > 20 Hz train of microV pulses in the bath, at levels from the primary medullary nucleus to the telencephalon. A precursor can be seen in the afferent nerve. The OSP follows the due-time of the first omitted stimulus with a, usually, constant main peak latency, 30-50 ms in medullary dorsal nucleus, 60-100 ms in midbrain, 120-190 ms in telencephalon-as though the brain has an expectation specific to the interstimulus interval (ISI). The latency, form and components vary between nerve, medulla, midbrain and forebrain. They include early fast waves, later slow waves and labile induced rhythms. Responsive loci are quite local. Besides ISI, which exerts a strong influence, many factors affect the OSP slightly, including train parameters and intensity, duration and polarity of the single stimulus pulses. Jitter of ISI does not reduce the OSP substantially, if the last interval equals the mean; the mean and the last interval have the main effect on both amplitude and latency. Taken together with our recent findings on visually evoked OSPs, we conclude that OSPs do not require higher brain levels or even the complexities of the retina. They appear in primary sensory nuclei and are then modified at midbrain and telencephalic levels. We propose that the initial processes are partly in the receptors and partly in the first central relay including a rapid increase of some depressing influence contributed by each stimulus. This influence comes to an ISI-specific equilibrium with the excitatory influence; withholding a stimulus and hence its depressing influence causes a rebound excitation with a specific latency.

Peripheral origin of olfactory nerve fibers by-passing the olfactory bulb in Xenopus laevis

After DiI injections into the diencephalon of Xenopus, two types of retrogradely labelled cells were found in the nasal area: (i) receptor cells in the olfactory epithelium and (ii) a small cell group located between the main olfactory epithelium and the vomeronasal system. These results reveal an extensive extrabulbar olfactory projection of olfactory receptor cells. Fibers of these cells do not terminate in the olfactory bulb but innervate targets in the diencephalon directly. The other type of retrogradely labelled cells, apparently, are not part of any epithelium. They resemble similar cell groups which have previously been regarded as part of the nervus terminalis system in other vertebrates.

Functional subdivisions of the olfactory system correlate with lectin-binding properties in Xenopus

Soybean agglutinin (SBA) is known to selectively label a portion of neurons in amphibian and mammalian primary olfactory systems. Hitherto, no other distinctive features have been found to correlate with the two neuronal populations. Investigating SBA-HRP binding in olfactory mucosa and CNS of Xenopus, we noted that labelled and unlabelled structures can readily be assigned to different olfactory subsystems. The SBA negative one is utilized to detect air-borne odors, whereas major SBA-positive structures serve a role in the perception of water dissolved molecules. Some labelled fibers by-pass the olfactory bulb, traverse the telencephalon and innervate prosencephalic structures. They are considered to be aberrant olfactory nerve fibers, rather than being part of the terminal nerve.

Subdivisions of the terminal nerve in Xenopus laevis

The majority of recent studies on the terminal nerve (nt) in various vertebrates either involved tracer injections into the nasal cavity or made use of the LHRH-/FMRFamide-like immunoreactivity (ir) of a portion of its fibers. The present investigation was designed to determine the extent of overlap between data rendered by the two methods in Xenopus. The findings reveal no overlap of nt projections visualized by the two experimental techniques. This result sheds doubt on the validity of current definitions of the nt system.

Histochemical, connectional and cytoarchitectonic evidence for a secondary reduction of the pretectum in the European eel, Anguilla anguilla: a case of parallel evolution

There are at least three different patterns of pretectal organization in teleost fishes: a simple pattern observed in cyprinids, an elaborate pattern present in percomorphs, and an intermediately complex pattern seen in many other teleost groups. The taxonomic distribution of the pretectal patterns indicates that the simple and the elaborate patterns are both evolutionarily derived (apomorphic) from the primitive (plesiomorphic) intermediately complex one. In anguillids, the pretectal pattern observed cytoarchitectonically has an anatomical configuration similar to that of the simple pattern in cyprinids. The distribution of acetylcholinesterase positivity in the pretectum (namely acetylcholinesterase positivity in the parvo- and magnocellular superficial and posterior pretectal nuclei, and acetylcholinesterase negativity in the pretectal cell plate and the ovoid preglomerular cell aggregate), as well as the retinal projections (namely retinal terminals in the parvocellular superficial and central pretectal nuclei, and absence of such terminals in the magnocellular superficial and posterior pretectal nuclei and the pretectal cell plate), strongly supports the interpretation suggested by the cytoarchitectonic analysis. As anguillids (elopomorpha) and cyprinids (ostariophysi) are related only distantly, this secondary simplification in the pretectum likely occurred independently, i.e. this simplification represents a case of parallel reduction.

Event-related potentials in the retina and optic tectum of fish

1. Compound field potentials were recorded with up to 18 microelectrodes in comb, brush, or spear arrays on and in the optic tectum and with suction electrodes from the distal stump of the cut optic nerve and from the optic nerve head in the opened eye in elasmobranchs and teleosts. Diffuse light flashes of different durations and submaximal intensities were delivered in trains with regular or irregular interstimulus intervals (ISI). 2. Event-related potentials (ERPs) are visible in single trials and begin at 50-200 ms after an "oddball" flash, especially one that is slightly weaker, briefer, or delayed by as little as 6% of ISI, compared with the more frequent stimulus. ERPs to the opposite condition are not of the same form or size. 3. One or more stimuli were omitted from a train or the train terminated after various conditioning times. Deflections occur beyond the expected visual-evoked potentials (VEPs) to the last flash and are called omitted-stimulus potentials (OSPs). They occur on schedule--approximately 100 ms after the next flash would be due--almost independent of intensity, duration, or conditioning time. They are considered to be ERPs without any necessary implication or denial of a temporally specific expectation. 4. Three components of OSP occur alone or in combination: an initial fast peak, a slow wave, and an oscillatory spindle up to ls or more in duration. This resembles the OFF response to steady light. 5. All these components are already present in the retina with optic nerve cut. 6. The same mean ISI with a high proportion of jitter gives OSPs with only slightly longer latencies and smaller amplitudes; the OSP acts as though the retina makes an integrated prediction of ISI, intensity, and duration. 7. During a conditioning train the equilibrium between excitation and inhibition after each flash changes according to frequency, intensity, duration, and conditioning time; the VEP reflects this in a shape unique to the ISI; inhibition increases rapidly after each flash and then decays slowly according to the recent mean ISI. This allows rebound disinhibition after missing, weak, or delayed flashes (OSP or ERP) or causes an altered VEP after a longer or stronger oddball. 8. It seems unlikely that the OSP or oddball ERP in fish tectum is equivalent to mammalian ERPs under the same regime or signals higher cognitive events, because they are already present in the retina, require flash frequencies greater than 1 Hz, and grow with frequency up to and beyond flicker fusion.(ABSTRACT TRUNCATED AT 400 WORDS)

Dynamic properties of visual evoked potentials in the tectum of cartilaginous and bony fishes, with neuroethological implications

We have extended the study of Bullock ('84), who reported on visually evoked potentials (VEP) in the tectum of 10 species of elasmobranchs, by adding further stimulus regimes, multichannel recording, and additional taxa, particularly addressing the question of flicker fusion frequency by electrophysiological signs in central processing centers. Using principally the guitarfish, Platyrhinoidis and Rhinobatos, and the bass, Paralabrax, with some additional data from 32 other species, the findings support the following conclusions: 1. Latency of the first main VEP peak, a sharp surface negativity, to a diffuse white flash of submaximal intensity while the eye is moderately light adapted varies from less than 20 ms in some teleosts to greater than 120 ms in agnathans, holocephalans, and some rays. Among the elasmobranchs tested, the sharks are generally faster than the rays. Among the teleosts tested, some species are at least three times slower than others. There is little overlap between the fastest elasmobranchs and the slowest teleosts. 2. After the first VEP peak, later components are more diverse than the classic descriptions of one late surface-negative hump; they may include also sharp peaks, slow humps, and oscillatory waves extending out to greater than 1 s postflash. These are highly labile, variable and similar to OFF responses after a long light pulse. All these components occur already in the retina, whether the optic nerve is intact or cut. Many records do not show the late components; in the same preparation, some tectal loci may and others may not. 3. Ongoing activity (the micro-EEG, over all frequency bands) is depressed between early and late waves after a flash as well as during a long light pulse. 4. Repeated flashes above a few per second do not so much cause fatigue of the VEPs as reduce or prevent them by a sustained inhibition; large late waves are released as a rebound excitation any time the train of flashes stops or is delayed or sufficiently weakened. 5. Repeated flashes depress first the early waves; later waves follow 1:1 up to an upper following frequency (UFF) of approximately 13 Hz in the guitarfishes at optimal intensity and light adaptation (15-17 degrees C). A transition zone of gradual fusion from 15 to 30 Hz is marked by sputtering or irregular sharp VEPs; above a lower fusion frequency (LFF) of 30-40 Hz, the flash train becomes equivalent to steady light.(ABSTRACT TRUNCATED AT 400 WORDS)

The nervus terminalis in larval and adult Xenopus laevis

Nervus terminalis (nt) projections were studied by HRP injections into one nostril in adult Xenopus and in Xenopus tadpoles. Central nt targets are: medial septum, preoptic nucleus, nucleus of the anterior commissure, and hypothalamus (mainly ipsilaterally). In Xenopus tadpoles, additional fibers reach the ipsilateral dorsal thalamus and the mesencephalic tegmentum, bilaterally; furthermore, hypothalamic projections are bilateral. Xenopus tadpole nt connections resemble those of adult urodeles more closely than the projections of frogs. However, Xenopus tadpoles lack nt innervation of the medial septum.