The SOS1 Inhibitor MRTX0902 Blocks KRAS Activation and Demonstrates Antitumor Activity in Cancers Dependent on KRAS Nucleotide Loading

KRAS is the most frequently mutated oncogene in human cancer and facilitates uncontrolled growth through hyperactivation of the RTK/MAPK pathway. The Son of Sevenless homolog 1 (SOS1) protein functions as a guanine nucleotide exchange factor (GEF) for the RAS subfamily of small GTPases and represents a druggable target in the pathway. Using a structure-based drug discovery approach, MRTX0902 was identified as a selective and potent SOS1 inhibitor that disrupts the KRAS:SOS1 protein-protein interaction to prevent SOS1-mediated nucleotide exchange on KRAS and translates into an anti-proliferative effect in cancer cell lines with genetic alterations of the KRAS-MAPK pathway. MRTX0902 augmented the antitumor activity of the KRAS G12C inhibitor adagrasib when dosed in combination in eight out of twelve KRAS G12C-mutant human non-small cell lung cancer (NSCLC) and colorectal cancer (CRC) xenograft models. Pharmacogenomic profiling in preclinical models identified cell cycle genes and the SOS2 homolog as genetic co-dependencies and implicated tumor suppressor genes (NF1, PTEN) in resistance following combination treatment. Lastly, combined vertical inhibition of RTK/MAPK pathway signaling by MRTX0902 with inhibitors of EGFR or RAF/MEK led to greater downregulation of pathway signaling and improved antitumor responses in KRAS-MAPK pathway-mutant models. These studies demonstrate the potential clinical application of dual inhibition of SOS1 and KRAS G12C and additional SOS1 combination strategies that will aide in the understanding of SOS1 and RTK/MAPK biology in targeted cancer therapy.

Discovery of Pyridopyrimidinones that Selectively Inhibit the H1047R PI3Kα Mutant Protein

The H1047R mutation of PIK3CA is highly prevalent in breast cancers and other solid tumors. Selectively targeting PI3KαH1047R over PI3KαWT is crucial due to the role that PI3KαWT plays in normal cellular processes, including glucose homeostasis. Currently, only one PI3KαH1047R-selective inhibitor has progressed into clinical trials, while three pan mutant (H1047R, H1047L, H1047Y, E542K, and E545K) selective PI3Kα inhibitors have also reached the clinical stage. Herein, we report the design and discovery of a series of pyridopyrimidinones that inhibit PI3KαH1047R with high selectivity over PI3KαWT, resulting in the discovery of compound 17. When dosed in the HCC1954 tumor model in mice, 17 provided tumor regressions and a clear pharmacodynamic response. X-ray cocrystal structures from several PI3Kα inhibitors were obtained, revealing three distinct binding modes within PI3KαH1047R including a previously reported cryptic pocket in the C-terminus of the kinase domain wherein we observe a ligand-induced interaction with Arg1047.

Stereoselective Amine Synthesis Mediated by a Zirconocene Hydride to Accelerate a Drug Discovery Program

Chiral amine synthesis remains a significant challenge in accelerating the design cycle of drug discovery programs. A zirconium hydride, due to its high oxophilicity and lower reactivity, gave highly chemo- and stereoselective reductions of sulfinyl ketimines. The development of this zirconocene-mediated reduction helped to accelerate our drug discovery efforts and is applicable to several motifs commonly used in medicinal chemistry. Computational investigation supported a cyclic half-chair transition state to rationalize the high selectivity in benzyl systems.

Discovery of Five SOS2 Fragment Hits with Binding Modes Determined by SOS2 X-Ray Cocrystallography

SOS1 and SOS2 are guanine nucleotide exchange factors that mediate RTK-stimulated RAS activation. Selective SOS1:KRAS PPI inhibitors are currently under clinical investigation, whereas there are no reports to date of SOS2:KRAS PPI inhibitors. SOS2 activity is implicated in MAPK rebound when divergent SOS1 mutant cell lines are treated with the SOS1 inhibitor BI-3406; therefore, SOS2:KRAS inhibitors are of therapeutic interest. In this report, we detail a fragment-based screening strategy to identify X-ray cocrystal structures of five diverse fragment hits bound to SOS2.

Optimization of mTOR Inhibitors Using Property-Based Drug Design and Free-Wilson Analysis for Improved In Vivo Efficacy

The mTOR kinase regulates a variety of critical cellular processes and has become a target for the treatment of various cancers. Using a combination of property-based drug design and Free-Wilson analysis, we further optimized a series of selective mTOR inhibitors based on the (S)-6a-methyl-6a,7,9,10-tetrahydro[1,4]oxazino[3,4-h]pteridin-6(5H)-one scaffold. Our efforts resulted in 14c, which showed similar in vivo efficacy compared to previous lead 1 at 1/15 the dose, a result of its improved drug-like properties.

MRTX1719 Is an MTA-Cooperative PRMT5 Inhibitor That Exhibits Synthetic Lethality in Preclinical Models and Patients with MTAP-Deleted Cancer

Previous studies implicated protein arginine methyltransferase 5 (PRMT5) as a synthetic lethal target for MTAP-deleted (MTAP del) cancers; however, the pharmacologic characterization of small-molecule inhibitors that recapitulate the synthetic lethal phenotype has not been described. MRTX1719 selectively inhibited PRMT5 in the presence of MTA, which is elevated in MTAP del cancers, and inhibited PRMT5-dependent activity and cell viability with >70-fold selecti-vity in HCT116 MTAP del compared with HCT116 MTAP wild-type (WT) cells. MRTX1719 demonstrated dose-dependent antitumor activity and inhibition of PRMT5-dependent SDMA modification in MTAP del tumors. In contrast, MRTX1719 demonstrated minimal effects on SDMA and viability in MTAP WT tumor xenografts or hematopoietic cells. MRTX1719 demonstrated marked antitumor activity across a panel of xenograft models at well-tolerated doses. Early signs of clinical activity were observed including objective responses in patients with MTAP del melanoma, gallbladder adenocarcinoma, mesothelioma, non-small cell lung cancer, and malignant peripheral nerve sheath tumors from the phase I/II study.

SIGNIFICANCE

PRMT5 was identified as a synthetic lethal target for MTAP del cancers; however, previous PRMT5 inhibitors do not selectively target this genotype. The differentiated binding mode of MRTX1719 leverages the elevated MTA in MTAP del cancers and represents a promising therapy for the ∼10% of patients with cancer with this biomarker. See related commentary by Mulvaney, p. 2310. This article is featured in Selected Articles from This Issue, p. 2293.

Fragment optimization and elaboration strategies - the discovery of two lead series of PRMT5/MTA inhibitors from five fragment hits

Here we describe the early stages of a fragment-based lead discovery (FBLD) project for a recently elucidated synthetic lethal target, the PRMT5/MTA complex, for the treatment of MTAP-deleted cancers. Starting with five fragment/PRMT5/MTA X-ray co-crystal structures, we employed a two-phase fragment elaboration process encompassing optimization of fragment hits and subsequent fragment growth to increase potency, assess synthetic tractability, and enable structure-based drug design. Two lead series were identified, one of which led to the discovery of the clinical candidate MRTX1719.

Design and evaluation of achiral, non-atropisomeric 4-(aminomethyl)phthalazin-1(2H)-one derivatives as novel PRMT5/MTA inhibitors

MRTX1719 is an inhibitor of the PRMT5/MTA complex and recently entered clinical trials for the treatment of MTAP-deleted cancers. MRTX1719 is a class 3 atropisomeric compound that requires a chiral synthesis or a chiral separation step in its preparation. Here, we report the SAR and medicinal chemistry design strategy, supported by structural insights from X-ray crystallography, to discover a class 1 atropisomeric compound from the same series that does not require a chiral synthesis or a chiral separation step in its preparation.

Anti-tumor efficacy of a potent and selective non-covalent KRASG12D inhibitor

Recent progress in targeting KRAS^G12C has provided both insight and inspiration for targeting alternative KRAS mutants. In this study, we evaluated the mechanism of action and anti-tumor efficacy of MRTX1133, a potent, selective and non-covalent KRAS^G12D inhibitor. MRTX1133 demonstrated a high-affinity interaction with GDP-loaded KRAS^G12D with K_D and IC₅₀ values of ~0.2 pM and <2 nM, respectively, and ~700-fold selectivity for binding to KRAS^G12D as compared to KRAS^WT. MRTX1133 also demonstrated potent inhibition of activated KRAS^G12D based on biochemical and co-crystal structural analyses. MRTX1133 inhibited ERK1/2 phosphorylation and cell viability in KRAS^G12D-mutant cell lines, with median IC₅₀ values of ~5 nM, and demonstrated >1,000-fold selectivity compared to KRAS^WT cell lines. MRTX1133 exhibited dose-dependent inhibition of KRAS-mediated signal transduction and marked tumor regression (≥30%) in a subset of KRAS^G12D-mutant cell-line-derived and patient-derived xenograft models, including eight of 11 (73%) pancreatic ductal adenocarcinoma (PDAC) models. Pharmacological and CRISPR-based screens demonstrated that co-targeting KRAS^G12D with putative feedback or bypass pathways, including EGFR or PI3Kα, led to enhanced anti-tumor activity. Together, these data indicate the feasibility of selectively targeting KRAS mutants with non-covalent, high-affinity small molecules and illustrate the therapeutic susceptibility and broad dependence of KRAS^G12D mutation-positive tumors on mutant KRAS for tumor cell growth and survival.

Atropisomeric Racemization Kinetics of MRTX1719 Using Chiral Solvating Agent-Assisted 19F NMR Spectroscopy

With renewed interest in atropisomerism of drug molecules, efficient methods to experimentally determine torsion rotational energy barriers are needed. Here, we describe use of the chiral phosphoric acid solvating agent (+)-TiPSY to resolve the signals of atropisomers in ¹⁹F NMR and to use the data to study the kinetics of racemization and determine the rotational energy barrier of clinical compound MRTX1719. This method is complimentary to traditional chiral high-performance liquid chromatography (HPLC) and enhances the toolkit for chiral analysis techniques.

Design and Discovery of MRTX0902, a Potent, Selective, Brain-Penetrant, and Orally Bioavailable Inhibitor of the SOS1:KRAS Protein-Protein Interaction

SOS1 is one of the major guanine nucleotide exchange factors that regulates the ability of KRAS to cycle through its “on” and “off” states. Disrupting the SOS1:KRAS^G12C protein–protein interaction (PPI) can increase the proportion of GDP-loaded KRAS^G12C, providing a strong mechanistic rationale for combining inhibitors of the SOS1:KRAS complex with inhibitors like MRTX849 that target GDP-loaded KRAS^G12C. In this report, we detail the design and discovery of MRTX0902—a potent, selective, brain-penetrant, and orally bioavailable SOS1 binder that disrupts the SOS1:KRAS^G12C PPI. Oral administration of MRTX0902 in combination with MRTX849 results in a significant increase in antitumor activity relative to that of either single agent, including tumor regressions in a subset of animals in the MIA PaCa-2 tumor mouse xenograft model.

Abstract LB505: Design and discovery of MRTX0902, a potent, selective, and orally bioavailable SOS1 inhibitor

KRAS mutations are the most common activating mutations in human cancer that ultimately lead to hyperactivation of the MAPK pathway and uncontrolled growth. KRAS functions as a small GTPase that cycles through its GTP-loaded “on” state and its GDP-loaded “off” state, a highly regulated process that is crucial for normal cell proliferation and survival. The guanine nucleotide exchange factor (GEF) SOS1 plays a critical role in this process by regulating the “on/off” state of KRAS. The protein-protein interaction between SOS1 and KRAS facilitates turnover of KRAS from the GDP-loaded inactive state to its activated and GTP-loaded state, a critical step to enable productive KRAS effector binding and activation of downstream signaling. The KRASG12C inhibitor, adagrasib (MRTX849), irreversibly binds to the GDP-loaded inactive conformation of KRASG12C and has recently shown encouraging clinical activity across several cancer types. As adagrasib binds preferentially to the inactive state of KRAS, blockade of SOS1 is anticipated to shift KRASG12C into the adagrasib-susceptible GDP-loaded state. Furthermore, this combination strategy could be used to target other mutant-driven cancers within the MAPK pathway using the appropriate KRASmut inhibitors and/or inhibitors of other targets within the MAPK pathway including MEK or EGFR. MRTX0902 was identified using iterative structure-based design as a selective inhibitor of SOS1 that demonstrates an IC50 value of 2 nM in a SOS1 HTRF binding assay and 30 nM in an MKN1 cellular assay. In pharmacokinetic evaluation across species, MRTX0902 demonstrated low extraction ratios and moderate to high bioavailability in mice, rats, and dogs. In preclinical models, MRTX0902 augmented the antitumor activity of adagrasib and other selected therapies. The design, discovery, and preclinical characterization of the potential best-in-class candidate MRTX0902 will be described.

Citation Format: John M. Ketcham, David M. Briere, Aaron C. Burns, James G. Christensen, Robin J. Gunn, Jacob Haling, Anthony Ivetac, Shilpi Khare, Jon Kuehler, Svitlana Kulyk, Jade Laguer, John D. Lawson, Krystal Moya, Natalie Nguyen, Peter Olson, Lisa Rahbaek, Christopher R. Smith, Niranjan Sudhakar, Nicole C. Thomas, Darin Vanderpool, Xiaolun Wang, Matthew A. Marx. Design and discovery of MRTX0902, a potent, selective, and orally bioavailable 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 LB505.

Abstract ND02: MRTX0902: A SOS1 inhibitor for therapeutic intervention of KRAS-driven cancers

KRAS is the most frequently mutated oncogene in cancer and drives uncontrolled growth through hyperactivation of the MAPK pathway. Significant progress has been made in the past several years to directly target KRASG12C with the FDA approval of sotorasib and the reported clinical activity of adagrasib (MRTX849). Despite these remarkable breakthroughs, additional therapies that enhance the depth and duration of response to KRASG12C inhibitors provide the opportunity to build upon the initial progress. SOS proteins are guanine nucleotide exchange factors (GEFs) that transduce receptor tyrosine kinase (RTK) signaling from the cell surface and facilitate the activation of RAS family proteins. In addition, SOS1 is a target of negative feedback signaling following RAS-mediated activation of the RAF-MEK-ERK cascade. Thus, SOS proteins represent a significant therapeutic node that maintains RAS pathway equilibrium as well as oncogenic signaling dynamics. Here we highlight the discovery and preclinical evaluation of MRTX0902, a potent, selective, and orally bioavailable inhibitor of SOS1 presently in IND-enabling studies. A structure-based approach was used to identify a novel chemical series that disrupts the protein-protein interaction between SOS1 and KRAS, thereby preventing SOS1-mediated GTP-exchange on GDP-bound KRAS. Considering MRTX849 preferentially binds to inactive GDP-bound KRASG12C, targeting SOS1 in this genetic context increases the ability of MRTX849 to bind and inhibit KRASG12C. The combination of MRTX0902 with MRTX849 enhances the depth and durability of an anti-tumor response when compared to MRTX849 alone in pre-clinical KRASG12C tumor models. MRTX0902 augments additional targeted therapies across a variety of RAS-addicted tumors, indicating that SOS1 inhibition is effective against a broad spectrum of mutations within the MAPK pathway. Furthermore, drug-anchored CRISPR experiments with MRTX0902 and MRTX849 uncovered a previously underappreciated functional role of the SOS1 paralog, SOS2, in KRAS-addicted tumors. In addition to aiding in the understanding of SOS and RAS family signaling dynamics, these studies implicate SOS2 as a potential cancer drug target in the context of SOS1/KRASG12C inhibition. In summary, we have used a structure-based approach to discover a SOS1 inhibitor that augments the anti-tumor activity of MRTX849 and additional targeted MAPK pathway inhibitors. We anticipate our findings to translate into the clinic and make an impact in patients with RAS-addicted tumors.

Citation Format: John M. Ketcham, Shilpi Khare, Niranjan Sudhakar, David M. Briere, Larry Yan, Jade Laguer, Laura Vegar, Darin Vanderpool, Jill Hallin, Lauren Hargis, Vickie Bowcut, David Lawson, Robin J. Gunn, Anthony Ivetac, Nicole C. Thomas, Barbara Saechao, Natalie Nguyen, Jeffrey Clarine, Lisa Rahbaek, Christopher R. Smith, Aaron C. Burns, Matthew A. Marx, James G. Christensen, Peter Olson, Jacob R. Haling. MRTX0902: A SOS1 inhibitor for therapeutic intervention of KRAS-driven cancers [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 ND02.

Abstract 1131: Pharmacogenomic insight into targetable vulnerabilities and modifiers of response to MRTX1133 in KRASG12D-mutant models

The ability to effectively target mutated KRAS has remained elusive despite decades of research. Recently, the identification of selective KRASG12C inhibitors has provided an effective treatment option for patients harboring this particular mutation and has also provided insight toward targeting other KRAS mutants, including KRASG12D. MRTX1133 was identified via a structure-based drug design strategy as a potent, selective, and non-covalent KRASG12D inhibitor directed at the switch II binding pocket. MRTX1133 exhibited dose dependent KRAS pathway and tumor growth inhibition and demonstrated marked anti-tumor activity across a panel of models in vivo, including tumor regression &gt;30% in 8 out of 11 pancreatic ductal adenocarcinoma (PDAC) models. While evaluation of MRTX1133 across a panel of cell and patient-derived xenograft models demonstrated strong single agent activity in the majority of models tested, a subset of models were less responsive. Focused sgRNA libraries targeting ~5,000 genes were tested in MRTX1133-anchored CRISPR/Cas9 screens in vitro and in vivo in PDAC and colorectal KRASG12D-mutant cell lines. Similar to what was seen in a targeted genetic screen with our selective KRASG12C inhibitor MRTX849, several genes that act either upstream or downstream of KRAS were depleted with MRTX1133 treatment which illuminate specific targetable vulnerabilities in the context of KRASG12D inhibition. sgRNAs targeting EGFR, PIK3CA, PTPN11, mTOR, and CDK2/4/6 were depleted in MRTX1133-treated (or vehicle-treated) cell lines or xenograft models, while hallmark tumor suppressor genes RB1, KEAP1, NF1 and PTEN were enriched. The enrichment of KEAP1 sgRNAs in PDAC models parallels findings with MRTX849 in preclinical models and is in line with emerging clinical data suggesting increased capacity to scavenge reactive oxygen species (ROS) may represent a mechanism of partial resistance to a non-covalent, non-electrophilic warhead-containing KRASG12D-selective inhibitor in PDAC. Treatment using small molecules targeting selected vulnerabilities in vitro and in vivo confirmed these genetic findings. Further evaluation of dependencies utilizing an integrated analysis of RNAseq data also revealed KRAS regulates and is critically dependent on pro-survival and cell cycle genes for cancer cell viability. These data lend further insight into tumor response to KRASG12D inhibition and provide key insight into the genes that mediate the mechanism of action of, as well as confer partial resistance to MRTX1133, and identify combination targets that can augment the anti-tumor effect of MRTX1133.

Citation Format: Vickie Bowcut, Andrew Calinisan, David M. Briere, Lauren Hargis, Lars D. Engstrom, Jade Laguer, Darin Vanderpool, Xiaolun Wang, J David Lawson, Nicole Thomas, Robin Gunn, Christopher R. Smith, Julio Fernandez-Banet, Leo He, Adam Pavlicek, Lisa Rahbaek, Matthew A. Marx, Peter Olson, James G. Christensen, Jill Hallin. Pharmacogenomic insight into targetable vulnerabilities and modifiers of response to MRTX1133 in KRASG12D-mutant models [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 1131.

Abstract LB193: Chemical genomics identify novel druggable nodes and resistance pathways in the presence of concomitant SOS1 and KRAS inhibition

KRAS is the most frequently mutated oncogene in human cancer and facilitates uncontrolled growth through hyperactivation of the MAPK pathway. Recent data has consistently demonstrated co-dependencies of mutant-KRAS with extrinsic proteins that augment GTP-loading. Son of Sevenless homolog 1 (SOS1) is the most proximal of these proteins to KRAS and functions as a guanine nucleotide exchange factor (GEF) for the RAS subfamily of small GTPases, thus representing a highly sought-after druggable target. Utilizing a structure-based drug discovery approach, we identified a selective and potent SOS1 inhibitor, MRTX0902, that functions by disrupting the KRAS/SOS1 protein-protein interaction, ultimately preventing SOS1-mediated nucleotide exchange on KRAS. MRTX0902 enhances the anti-tumor activity of the KRASG12C inhibitor adagrasib across multiple KRASG12C-mutant preclinical models; however, it is anticipated that deeper pharmacological inhibition of KRAS signaling may give rise to novel combination targets and additional adaptive resistance mechanisms. To identify additional vulnerabilities and combination strategies, we conducted drug-anchored CRISPR screens in two KRASG12C xenograft models in the presence of MRTX0902 and adagrasib in vitro and in vivo. Interestingly, we found that the SOS1 homolog, SOS2, only emerges as a co-dependency under selective pressure of combination treatment and likely compensates for the inhibition of SOS1. Furthermore, multiple components from the PIK3CA/mTOR signaling pathway, PRMT5, and other non-vertical signal transduction pathways were uncovered and identified as genetic co-dependencies that emerged under selective pressure. Conversely, tumor suppressor genes including TSC1/2, PTEN, NF1, KEAP1 and TP53 along with phosphatases PTPN12 and PTPN14 enhanced tumor growth when knocked out providing a catalogue of putative resistance genes and mechanisms. Lastly, we utilized small molecule inhibitors of putative therapeutic targets identified from our CRISPR screens to validate genetic co-dependencies across in vitro and in vivo translational models. These studies uncover the potential utility of additional drug partners for the MRTX0902 and adagrasib combination and aide in the understanding of SOS and RAS biology in targeted cancer therapy.

Citation Format: Shilpi Khare, Niranjan Sudhakar, David M. Briere, Larry Yan, Lars D. Engstrom, Jade Laguer, James Medwid, Laura Vegar, Darin Vanderpool, Matthew A. Marx, John M. Ketcham, James G. Christensen, Peter Olson, Jacob R. Haling. Chemical genomics identify novel druggable nodes and resistance pathways in the presence of concomitant SOS1 and KRAS inhibition [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 LB193.

Abstract 1841: MRTX849 inhibits P-gp and demonstrates CNS exposure in mouse models and cancer patients and demonstrates antitumor activity in intracranial mouse models of lung cancer brain metastasis

MRTX849 is a covalent, mutant selective, KRAS G12C inhibitor in development for cancer patients harboring this mutation. KRAS G12C is mutated in ~14% of lung adenocarcinoma and KRAS G12C inhibitors adagrasib and sotorasib have demonstrated clinical activity. While early data are encouraging, approximately one-third of KRAS-mutant non-small cell lung cancer (NSCLC) patients develop brain metastases and this remains a significant unmet medical need. P-glycoprotein 1 (P-gp)-mediated efflux is a major mechanism for the active transport of small molecules out of the CNS. MRTX849 is a P-gp inhibitor and inhibits its own efflux at plasma exposure levels achieved in humans at the 600 mg BID dose level resulting in achieving appreciable drug levels in cerebrospinal fluid (CSF). Total and free-fraction adjusted plasma concentrations of MRTX849 were 8.6 μM and 43 nM; respectively, 8 hours post administration of a clinically relevant oral dose of 200 mg/kg to mice. In addition, the CSF concentration, a measure of the free brain concentration, was 52 nM, which is above the cellular pERK1/2 IC50 value measured at 24 hours in cancer cell lines (~5 nM). Administration of 100 mg/kg MRTX849 to mice also resulted in CSF exposure above the cellular IC50 and a CSF/plasma (free-fraction adjusted) partition coefficient (Kp,uu) value of 0.4. To evaluate the tumor growth inhibition following oral administration of MRTX849 in an orthotopic model of lung cancer brain metastasis, immunocompromised mice were intracranially implanted with luciferase-labeled human NSCLC LU99 cells. Lower bioluminescence imaging (BLI)-based tumor flux was observed for the 100 mg/kg BID MRTX849-treated mice compared to vehicle and an 88% reduction in bioluminescence signal compared to baseline was observed suggesting strong tumor regression and consistent with significantly longer survival in treated mice. In addition, MRTX849 treated brain tumors demonstrated 85% reduced ERK phosphorylation. Similarly, treatment with MRTX849 resulted in potent tumor regression and significant survival extension in another intracranial KRAS G12C model of NSCLC using LU65 cells. CSF levels were determined at steady-state in two patients in the MRTX849-001 Phase1/2 clinical trial. The CSF/plasma (free fraction adjusted) Kp,uu value was 0.47 with patient CSF levels achieved consistent with CSF levels observed in responding mouse models (24-35 nM). These data demonstrate MRTX849 crosses the blood brain barrier in preclinical models and cancer patients. In addition, antitumor activity observed in mouse models of brain metastases provides rationale for exploring the utility of MRTX849 for the treatment of patients harboring KRAS G12C mutant lung cancer with brain metastases.

Citation Format: Kazuhide Shimizu, Jill Hallin, Mohini Singh, Naema Nayyar, Matthew R. Strickland, Aaron C. Burns, Cornelius Cilliers, Lauren Hargis, Peter A. Olson, Matthew A. Marx, Priscilla K. Brastianos, Hiroaki Wakimoto, James G. Christensen. MRTX849 inhibits P-gp and demonstrates CNS exposure in mouse models and cancer patients and demonstrates antitumor activity in intracranial mouse models of lung cancer brain metastasis [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 1841.

Fragment-Based Discovery of MRTX1719, a Synthetic Lethal Inhibitor of the PRMT5•MTA Complex for the Treatment of MTAP-Deleted Cancers

The PRMT5•MTA complex has recently emerged as a new synthetically lethal drug target for the treatment of MTAP-deleted cancers. Here, we report the discovery of development candidate MRTX1719. MRTX1719 is a potent and selective binder to the PRMT5•MTA complex and selectively inhibits PRMT5 activity in MTAP-deleted cells compared to MTAP-wild-type cells. Daily oral administration of MRTX1719 to tumor xenograft-bearing mice demonstrated dose-dependent inhibition of PRMT5-dependent symmetric dimethylarginine protein modification in MTAP-deleted tumors that correlated with antitumor activity. A 4-(aminomethyl)phthalazin-1(2H)-one hit was identified through a fragment-based screen, followed by X-ray crystallography, to confirm binding to the PRMT5•MTA complex. Fragment growth supported by structural insights from X-ray crystallography coupled with optimization of pharmacokinetic properties aided the discovery of development candidate MRTX1719.

Identification of MRTX1133, a Noncovalent, Potent, and Selective KRASG12D Inhibitor

KRAS^G12D, the most common oncogenic KRAS mutation, is a promising target for the treatment of solid tumors. However, when compared to KRAS^G12C, selective inhibition of KRAS^G12D presents a significant challenge due to the requirement of inhibitors to bind KRAS^G12D with high enough affinity to obviate the need for covalent interactions with the mutant KRAS protein. Here, we report the discovery and characterization of the first noncovalent, potent, and selective KRAS^G12D inhibitor, MRTX1133, which was discovered through an extensive structure-based activity improvement and shown to be efficacious in a KRAS^G12D mutant xenograft mouse tumor model.

Abstract P165: MRTX1719: A first-in-class MTA-cooperative PRMT5 inhibitor that selectively elicits antitumor activity in MTAP/CDKN2A deleted cancer models

Nearly all oncology therapies target proteins that are hyperactivated in cancer; however, developing precision medicines for cancers that harbor a specific tumor suppressor gene loss remains one of the most significant challenges in cancer research. Previous studies demonstrated cancer cell lines harboring homozygous deletion of the MTAP gene (MTAP del) are selectively vulnerable to shRNA-mediated PRMT5 inhibition based on the concept of synthetic lethality. MTAP is adjacent to, and co-deleted with, the most commonly deleted tumor suppressor gene, CDKN2A. MTAP is deleted in ~10% of all cancers and is enriched in NSCLC, pancreatic cancer, mesothelioma, MPNST, and several other cancers. MTAP del cells accumulate the metabolite methylthioadenosine (MTA) which binds to, and partially inhibits, PRMT5 activity; however, current clinical-stage small molecule PRMT5 inhibitors have binding modes that are mutually exclusive with MTA binding and therefore are not selective for MTAP del cancers. To develop a potential precision medicine for MTAP del cancers, compounds that selectively bind to the PRMT5/MTA complex were discovered via a fragment-based approach using SPR complemented with structural insight utilizing X-ray crystallography. Optimization of the lead series identified MRTX1719, a potent inhibitor of PRMT5 activity (8 nM IC50) and cell viability (12 nM IC50) in HCT116 MTAP del cells with greater than 50-fold selectivity compared with HCT116 MTAP WT cells. MRTX1719 possesses favorable drug-like characteristics and human predicted oral PK properties. Daily oral administration of MRTX1719 to tumor xenograft-bearing mice demonstrated dose-dependent inhibition of PRMT5-dependent symmetric dimethylarginine (SDMA) protein modification in MTAP del tumors that correlated with antitumor activity. In contrast, minimal inhibition of PRMT5-dependent SDMA and tumor growth inhibition was observed in MTAP WT tumor xenografts. Repeat dosing in mice also showed reduced effects in bone marrow compared to nonselective PRMT5 inhibitors currently in clinical trials, suggesting MRTX1719 may have an improved therapeutic index relative to these agents. MRTX1719 treatment resulted in dysregulated post-translational modification of key cancer and cell growth-related proteins, dysregulated RNA splicing, decreased proliferation and increased apoptosis in MTAP del cancer cells. Screens evaluating the effect of MRTX1719 on cell viability across panels of cancer cell lines and/or patient-derived xenograft tumor models demonstrated broad anti-tumor activity across MTAP-deleted tumor models. Select targeted therapy and chemotherapy combinations augmented the activity of MRTX1719 and in some models converged on phospho-Rb inhibition to block tumor growth. Together, these data demonstrate that MRTX1719, a selective PRMT5/MTA inhibitor, represents a promising therapeutic strategy for patients with MTAP del cancers.

Citation Format: Christopher R. Smith, Lars D. Engstrom, Svitlana Kulyk, Ruth Aranda, Laura Waters, Krystal Moya, Victoria Bowcut, Allan Hebbert, David Trinh, David M. Briere, J. David Lawson, Jeff Clarine, Lisa Rahbaek, James G. Christensen, Matthew A. Marx, Peter Olson. MRTX1719: A first-in-class MTA-cooperative PRMT5 inhibitor that selectively elicits antitumor activity in MTAP/CDKN2A deleted cancer models [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P165.

Abstract 1131: ORIC-944, a potent and selective allosteric PRC2 inhibitor, demonstrates robust in vivo activity in prostate cancer models

The polycomb repressive complex 2 (PRC2) is responsible for the methylation of histone 3 at lysine 27 (H3K27) which leads to long-term transcriptional silencing. Through this epigenetic chromatin silencing mechanism, PRC2 plays a key role in regulating cellular functions such as cell growth and differentiation. PRC2 comprises three core subunits: the catalytic subunit enhancer of zeste homolog 2 (EZH2), embryonic ectoderm development (EED) and suppressor of zeste 12 (SUZ12). EED directly interacts with histone H3K27me3 and is essential for the histone methyltransferase activity of PRC2. PRC2 dysregulation occurs in multiple solid tumors and hematological malignancies, resulting in elevated levels of PRC2 activity and H3K27 trimethylation, and has been linked to poor prognosis in patients with metastatic prostate cancer. First-generation PRC2 inhibitors which target EZH2 have demonstrated clinical activity in several cancers, yet the pharmacological and ADME properties of these compounds require high doses that only achieve partial target inhibition at clinically active levels and exhibit drug-drug interaction (DDI) liabilities.

As an alternative strategy to fully inhibit the PRC2 complex, we developed ORIC-944, a potent and highly selective allosteric inhibitor of PRC2 via binding the EED subunit. This unique EED targeting strategy can more completely inhibit PRC2, for example, in the presence of innate or acquired resistance mutations in EZH2, and by addressing the potential compensatory escape mechanism of EZH1-driven tumor growth. ORIC-944 has potential best-in-class drug properties compared to first generation PRC2 inhibitors, including superior potency and improved DDI liabilities. In diffuse large B-cell lymphoma (DLBCL) xenografts in vivo, ORIC-944 significantly depleted H3K27 trimethylation and induced tumor regressions in a dose-dependent manner.

ORIC-944 demonstrated strong tumor growth inhibition as a single agent with once daily oral dosing in both enzalutamide-responsive and enzalutamide-resistant prostate cancer models. ORIC-944 caused a significant reduction in tumor cell proliferation and anti-apoptotic signaling, as measured by Ki67 and survivin, respectively. Moreover, in PK/PD assessments, ORIC-944 strongly depleted H3K27me3 in treated tumors. These in vivo studies established that effective single agent inhibition of PRC2 via EED results in decreased tumor cell growth in PRC2-dependent prostate cancer models.

In summary, ORIC-944 is a potent, highly selective, allosteric, orally bioavailable PRC2 inhibitor via the EED subunit that represents a differentiated strategy to block PRC2 activity in selected cancers. We are developing ORIC-944 as a best-in-class PRC2 inhibitor for the treatment of patients with advanced prostate cancer and expect to file an IND in the second half of 2021.

Citation Format: Anneleen Daemen, Jessica D. Sun, Aleksandr Pankov, Frank L. Duong, Natalie Yuen, Shravani Barkund, Shelly Kaushik, Jae H. Chang, David M. Briere, Niranjan Sudhakar, Andrew Calinisan, Aaron Burns, John M. Ketcham, Matthew A. Marx, Peter Olson, James G. Christensen, Melissa R. Junttila, Lori S. Friedman. ORIC-944, a potent and selective allosteric PRC2 inhibitor, demonstrates robust in vivo activity in prostate cancer models [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 1131.

Abstract LB003: Fragment based discovery of MRTX9768, a synthetic lethal-based inhibitor designed to bind the PRMT5-MTA complex and selectively target MTAP/CDKN2A-deleted tumors

The MTAP gene is proximal to and co-deleted in nearly all CDKN2A-deleted cancers. This genetic alteration is present in an estimated 9% of all cancers and is especially prevalent in cancers with high unmet medical need (e.g. mesothelioma (32%), pancreatic (22%), lung squamous (20%)). Multiple independent research teams have demonstrated that tumor cell lines with homozygous MTAP deletions are hypersensitive to shRNA-mediated knock down of PRMT5. MTAP is required for the methionine salvage pathway and MTAP-del cells accumulate MTA, an inhibitory co-factor which competes for binding to the co-factor binding site of PRMT5 with the activating co-factor SAM. PRMT5 is a methyltransferase that adds symmetric dimethylarginine (SDMA) modification to proteins and is essential for mammalian cell survival. A small molecule that selectively binds and stabilizes the catalytically inactive PRMT5•MTA complex may represent a synthetic lethal-based precision medicine for the treatment of MTAP/CDKN2A—del tumors. Notably, 1st generation PRMT5 small molecule inhibitors do not target MTA-complexed PRMT5 and do not exhibit selective inhibition of MTAP-del cancer cells. Here we report a new series of compounds discovered via a fragment-based approach that selectively bind to the PRMT5•MTA complex. A fragment hit was identified in an SPR binding assay with PRMT5•MTA (KD 18 μM). The binding mode was determined by X-ray crystallography and revealed that the fragment makes productive interactions with K333, F327, E435, E444, E435, and W579 of PRMT5 as well as with the co-liganded MTA. Fragment growing aided by structure-based design identified a key interaction with the L312 backbone N-H that enhances binding to PRMT5•MTA (MRTX4646, SPR KD 57 nM). Further exploration highlighted an interaction with the F580 backbone N-H as important for cellular activity and selectivity. This interaction with F580 was illustrated by MRTX7512 which exhibits an IC50 value of 633 nM for inhibition of SDMA in engineered HCT116 MTAP-del cells and demonstrates 15-fold selectivity compared with HCT116 MTAP-WT cells (IC50 9763 nM). Further optimization of cellular potency and pharmacokinetic properties identified MRTX9768, a potent inhibitor of SDMA and cell proliferation in HCT116 MTAP-del cells (SDMA IC50 3 nM; prolif. IC50 11 nM) with marked selectivity over HCT116 MTAP-WT cells (SDMA IC50 544 nM; prolif. IC50 861 nM). In xenograft studies, oral administration of MRTX9768 demonstrates dose-dependent inhibition of SDMA in MTAP-del tumors, with less SDMA modulation observed in bone marrow. In summary, we have used a fragment-based approach to discover a new class of orally active PRMT5•MTA inhibitors that demonstrate selective antitumor activity in MTAP-del tumor cells while sparing MTAP-WT cells.

Citation Format: Christopher R. Smith, Svitlana Kulyk, J. D. Lawson, Lars D. Engstrom, Ruth Aranda, David M. Briere, Robin Gunn, Krystal Moya, Lisa Rahbaek, Laura Waters, Anthony Ivetac, James G. Christensen, Peter Olson, Matthew A. Marx. Fragment based discovery of MRTX9768, a synthetic lethal-based inhibitor designed to bind the PRMT5-MTA complex and selectively target MTAP/CDKN2A-deleted tumors [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 LB003.

Abstract LB-098: The anti-tumor activity of the KRAS G12C inhibitor MRTX849 is augmented by cetuximab in CRC tumor models

The ability to develop effective therapies for KRAS mutant cancers has remained elusive despite decades of research. MRTX849 was identified as a potent, selective, and covalent KRASG12C inhibitor presently under evaluation in clinical trials. MRTX849 inhibits only inactive GDP-bound KRASG12C, a mutation known to be dependent on nucleotide cycling and extrinsic factors which regulate the activation state of KRAS. Preclinical and early clinical data suggest KRASG12C inhibitors induce tumor regression/response in a subset of non-small cell lung cancer (NSCLC) and colorectal cancer (CRC) models and patients. Previous work in tumor xenograft models demonstrated activation of pathways upstream or downstream of KRAS through additional mutations or relief of feedback inhibition can limit the single agent activity of MRTX849 in the less responsive tumors. These data indicated that activation of receptor tyrosine kinases (RTKs) through relief of feedback inhibition was one mechanism implicated in modulating the response to MRTX849. As KRAS G12C mutations are present in ~4% of colon cancer, MRTX849 was further evaluated in CRC models to identify combination strategies that might optimize anti-tumor activity. A range of responses to single agent MRTX849 were observed across cell line and patient-derived xenograft (PDX) models. The combination of MRTX849 plus cetuximab demonstrated the best anti-tumor activity in multiple models. The combination of MRTX849 and cetuximab demonstrated increased KRASG12C modification and MAPK pathway inhibition suggesting these therapies converge to more fully inhibit KRASG12C. Despite the fact that cetuximab-based therapy is contra-indicated for KRAS-mutant CRC, these data suggest co-targeting EGFR and KRAS may more comprehensively inhibit KRAS mediated signaling and more effectively treat a subset of CRC patients with KRASG12C-mutant cancers.

Citation Format: Jill Hallin, Andrew Calinisan, Lauren Hargis, Ruth Aranda, Lars D. Engstrom, David M. Briere, Matthew A. Marx, Peter Olson, James G. Christensen. The anti-tumor activity of the KRAS G12C inhibitor MRTX849 is augmented by cetuximab in CRC tumor models [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr LB-098.

The KRASG12C Inhibitor MRTX849 Provides Insight toward Therapeutic Susceptibility of KRAS-Mutant Cancers in Mouse Models and Patients

Despite decades of research, efforts to directly target KRAS have been challenging. MRTX849 was identified as a potent, selective, and covalent KRAS^G12C inhibitor that exhibits favorable drug-like properties, selectively modifies mutant cysteine 12 in GDP-bound KRAS^G12C, and inhibits KRAS-dependent signaling. MRTX849 demonstrated pronounced tumor regression in 17 of 26 (65%) KRAS^G12C-positive cell line- and patient-derived xenograft models from multiple tumor types, and objective responses have been observed in patients with KRAS^G12C-positive lung and colon adenocarcinomas. Comprehensive pharmacodynamic and pharmacogenomic profiling in sensitive and partially resistant nonclinical models identified mechanisms implicated in limiting antitumor activity including KRAS nucleotide cycling and pathways that induce feedback reactivation and/or bypass KRAS dependence. These factors included activation of receptor tyrosine kinases (RTK), bypass of KRAS dependence, and genetic dysregulation of cell cycle. Combinations of MRTX849 with agents that target RTKs, mTOR, or cell cycle demonstrated enhanced response and marked tumor regression in several tumor models, including MRTX849-refractory models. SIGNIFICANCE: The discovery of MRTX849 provides a long-awaited opportunity to selectively target KRAS^G12C in patients. The in-depth characterization of MRTX849 activity, elucidation of response and resistance mechanisms, and identification of effective combinations provide new insight toward KRAS dependence and the rational development of this class of agents.See related commentary by Klempner and Hata, p. 20.This article is highlighted in the In This Issue feature, p. 1.

Abstract LB-271: Insight towards therapeutic susceptibility of KRAS mutant cancers from MRTX1257, a novel KRAS G12C mutant selective small molecule inhibitor

The ability to effectively target mutated KRAS has remained elusive despite decades of research. By solving a series of co-crystal structures coupled with iterative structure-based drug design, substituted tetrahydropyridopyrimidines were identified as selective, covalent inhibitors of mutant KRAS G12C. MRTX1257 emerged as a research tool compound that demonstrates the ability to irreversibly modify KRAS G12C, trap it in its inactive GDP-bound state, and inhibit ERK1/2 with an IC50 value of 1 nM. Therefore, studies were designed to provide insight towards the breadth of therapeutic response and the underlying molecular mechanisms of MRTX1257 activity. The evaluation of an extended panel of KRAS G12C mutant cell lines in cell viability assays indicated broad anti-tumor activity with a variable concentration-response pattern (0.2-62 nM). MRTX1257 also demonstrated dose-dependent irreversible modification of KRAS G12C and inhibition of KRAS-dependent signal transduction in multiple KRAS G12C mutant tumor models. MRTX1257 was then evaluated at a fixed dose level which demonstrated near-complete KRAS target inhibition with broad spectrum anti-tumor activity including deep tumor regressions in approximately 80% of all models evaluated across a large panel of KRAS G12C-mutant cell-derived and patient-derived xenografts (n = 23). MRTX1257 was inactive in non-KRAS G12C-mutant cell lines in vitro and in vivo. The antitumor response across tumor models varied in KRAS G12C positive models from durable complete tumor regression to tumor stasis. A small subset of models demonstrated rapid initial tumor regression, followed by tumor stasis suggesting that there may be mechanisms of adaptive tolerance to MRTX1257 treatment. Based on this response pattern, signal transduction and feedback signaling pathways were evaluated. In a temporal pattern consistent with drug tolerance kinetics, MAP kinase pathway reactivation was observed following continuous treatment based on rebound of pERK and pS6 signaling. Also consistent with signaling rebound dynamics and drug tolerance, down-regulation of dual specificity phosphatases (DUSPs), Sprouty family (SPRY), and ETVs along with decoupling of cell cycle from KRAS regulatory constraints was observed. CRISPR and small molecule combination screens identified mTOR, SHP2, EGFR family, and cell cycle kinases as targetable vulnerabilities in partially tolerant KRAS G12C mutant models. Based on these observations, a number of combination strategies designed to co-target signaling feedback pathways were evaluated in tumor models and provided insight into the importance of rebound pathways. Together, these data indicate the therapeutic susceptibility and broad dependence of KRAS G12C mutation-positive tumors and provide insight toward the molecular basis of response to single agent and combinatorial therapies.

Citation Format: James G. Christensen, Jay B. Fell, Matthew A. Marx, John Fischer, Jill Hallin, Andrew Calinisan, Brian Baer, Michael Burkhard, James Blake, Guy Vigers, Ruth Aranda, Lauren Hargis, David Briere, Lars Engstrom, Peter Olson. Insight towards therapeutic susceptibility of KRAS mutant cancers from MRTX1257, a novel KRAS G12C mutant selective small molecule inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-271.

Discovery of Tetrahydropyridopyrimidines as Irreversible Covalent Inhibitors of KRAS-G12C with In Vivo Activity

KRAS is the most frequently mutated driver oncogene in human cancer, and KRAS mutations are commonly associated with poor prognosis and resistance to standard treatment. The ability to effectively target and block the function of mutated KRAS has remained elusive despite decades of research. Recent findings have demonstrated that directly targeting KRAS-G12C with electrophilic small molecules that covalently modify the mutated codon 12 cysteine is feasible. We have discovered a series of tetrahydropyridopyrimidines as irreversible covalent inhibitors of KRAS-G12C with in vivo activity. The PK/PD and efficacy of compound 13 will be highlighted.

Discovery of the Highly Potent PI3K/mTOR Dual Inhibitor PF-04979064 through Structure-Based Drug Design

PI3K, AKT, and mTOR are key kinases from PI3K signaling pathway being extensively pursued to treat a variety of cancers in oncology. To search for a structurally differentiated back-up candidate to PF-04691502, which is currently in phase I/II clinical trials for treating solid tumors, a lead optimization effort was carried out with a tricyclic imidazo[1,5]naphthyridine series. Integration of structure-based drug design and physical properties-based optimization yielded a potent and selective PI3K/mTOR dual kinase inhibitor PF-04979064. This manuscript discusses the lead optimization for the tricyclic series, which both improved the in vitro potency and addressed a number of ADMET issues including high metabolic clearance mediated by both P450 and aldehyde oxidase (AO), poor permeability, and poor solubility. An empirical scaling tool was developed to predict human clearance from in vitro human liver S9 assay data for tricyclic derivatives that were AO substrates.

Discovery of novel, potent, and selective inhibitors of 3-phosphoinositide-dependent kinase (PDK1)

Analogues substituted with various amines at the 6-position of the pyrazine ring on (4-amino-7-isopropyl-7H-pyrrolo[2,3-d]pyrimidin-5-yl)pyrazin-2-ylmethanone were discovered as potent and selective inhibitors of PDK1 with potential as anticancer agents. An early lead with 2-pyridine-3-ylethylamine as the pyrazine substituent showed moderate potency and selectivity. Structure-based drug design led to improved potency and selectivity against PI3Kα through a combination of cyclizing the ethylene spacer into a saturated, five-membered ring and substituting on the 4-position of the aryl ring with a fluorine. ADME properties were improved by lowering the lipophilicity with heteroatom replacements in the saturated, five-membered ring. The optimized analogues have a PDK1 Ki of 1 nM and >100-fold selectivity against PI3K/AKT-pathway kinases. The cellular potency of these analogues was assessed by the inhibition of AKT phosphorylation (T308) and by their antiproliferation activity against a number of tumor cell lines.

Highly Selective and Potent Thiophenes as PI3K Inhibitors with Oral Antitumor Activity

Highly selective PI3K inhibitors with subnanomolar PI3Kα potency and greater than 7000-fold selectivity against mTOR kinase were discovered through structure-based drug design (SBDD). These tetra-substituted thiophenes were also demonstrated to have good in vitro cellular potency and good in vivo oral antitumor activity in a mouse PI3K driven NCI-H1975 xenograft tumor model. Compounds with the desired human PK predictions and good in vitro ADMET properties were also identified. In this communication, we describe the rationale behind the installation of a critical triazole moiety to maintain the intricate H-bonding network within the PI3K receptor leading to both better potency and selectivity. Furthermore, optimization of the C-4 phenyl group was exploited to maximize the compounds mTOR selectivity.

4-methylpteridinones as orally active and selective PI3K/mTOR dual inhibitors

Pteridinones were designed based on a non-selective kinase template. Because of the uniqueness of the PI3K and mTOR binding pockets, a methyl group was introduced to C-4 position of the peteridinone core to give compounds with excellent selectivity for PI3K and mTOR. This series of compounds were further optimized to improve their potency against PI3Kα and mTOR. Finally, orally active compounds with improved solubility and robust in vivo efficacy in tumor growth inhibition were identified as well.

Abstract 3484: In vivo activity of combined PI3k/mTOR and MEK-inhibition in a K-RASG12D; PTEN deletion mouse model of ovarian cancer

The PI3K/Akt pathway is commonly deregulated in human cancer, making it an attractive target for novel anti-cancer therapeutics. We have utilized a mouse model of ovarian cancer generated by K-RASG12D activation and PTEN deletion in the ovarian surface epithelium (Dinulescu, et al, 2005) for the pre-clinical assessment of a novel PI3K/mTOR inhibitor (PF-04691502, Pfizer). To enable high throughput studies, we have transplanted primary tumours from these mice orthotopically into the ovarian bursa.

Tumour growth rates were monitored by small animal-Ultrasound, and when tumours reached an average of 200mm3, mice received vehicle or 10mg/kg PF-04691502 daily for 7 days. Ultrasound and FDG-PET scans were performed on day 0, day 2 and day 7 of therapy. PF-04691502 inhibited tumour growth at 7 days by 71.64%±8.93. Tumour weights measured post-mortem following 7 days treatment also showed a significant decrease in tumour mass in treated mice (0.78±0.13g in vehicle treated versus 0.38±0.06g in drug treated mice; p&lt;0.05). FDG-PET imaging revealed that PF-04691502 reduced glucose metabolism dramatically at 48 hours, and this reduction was maintained for the 7 day treatment duration, suggesting FDG-PET may be exploited as a biomarker of response to PF-04691502. To confirm that the PI3K/mTOR pathway is successfully inhibited with PF-04691502, western blots and immunohistochemistry were performed on tumour samples harvested at 7 days post-treatment. These show that pAKT(S473) and pS6(S240/244) were dramatically inhibited following PF-04691502 treatment.

PF-04691502 was sufficient to inhibit PI3K/mTOR resulting in inhibition of tumour growth; however, we did not observe tumour regression in this model. Notably, tumours from PF-04691502 treated mice displayed activation of pERK, suggesting that activation of the MAPK pathway is a mechanism by which KRAS-mutant tumours can limit the efficacy of PI3K/mTOR inhibitors alone. To overcome the effects of RAS/MAPK signalling, we have performed studies evaluating PF-04691502 (7.5mg/kg, daily) in combination with an inhibitor of MEK (PD-0325901, Pfizer, 10mg/kg, daily). Ultrasound monitoring of tumour volume shows that over a 7 day treatment period, PF-04691502 led to tumour growth inhibition of 55%±10.27, PD-0325901 alone led to tumour regression (36.3%±20.45) and combined PD-0325901 and PF-04691502 led to a striking tumour regression of 80.7%±4.97. These data show that combined inhibition of PI3K/mTOR and MEK converts tumor growth delay with PI3K-inhibition alone to tumour regression in this KRAS and PTEN mutant mouse model of ovarian cancer. Therefore inhibition of MEK may enhance activity of PI3K/mTOR-inhibitors in tumours showing activation of the PI3K pathway by genomic changes such as mutation in KRAS combined with loss of PTEN.

Dinulescu D. M., et al. (2005) Nat. Med. 11:63-70.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3484.

Abstract 4482: Novel and selective small molecule inhibitors of 3-phosphoinositide-dependent kinase-1 inhibit the PDK-1/AKT signaling pathway and cell proliferation

Receptor tyrosine kinases (RTKs), PTEN (phosphatase and tensin homolog deleted on chromosome 10), and PIK3CA (encodes the p110α subunit of phosphatidylinositol 3-kinase [PI3K]) frequently contribute to tumor progression through their ability to regulate the intracellular level of phosphatidylinositol-3,4,5-triphosphate (PIP3). 3-phosphoinositide-dependent kinase-1 (PDK-1), a serine/threonine kinase, activates the catalytic domain of numerous kinases by phosphorylating their T-loop sites. PDK-1 activity is required for activation of AKT, p70S6K, and RSK which lead to cell proliferation and transformation. The interaction of the pleckstrin homology (PH) domain of AKT with the membrane bound PIP3 confers a conformational change in AKT, allowing PDK-1 to phosphorylate AKT at the residue threonine-308 (T308). This T-loop activation at T308, along with the phosphorylation of the serine-473 residue by mTOR, fully activates the AKT pathway. Although the roles of many PDK-1 substrates have yet to be characterized, the oncogenic activity of aberrant PI3K pathway signaling through PDK-1 has been extensively validated. We have developed a series of 3-Carbonyl-4-Amino-Pyrrolopyrimidne (CAP) compounds that are potent inhibitors of human PDK-1 (full length and kinase domain) which demonstrate more than 100-fold selectivity against P70S6K, PI3K, AKT, and mTOR. In this study, representative compounds from the CAP series were used to perform a variety of anti-tumor assays. We demonstrate PDK-1 compounds inhibit the phosphorylation of T308 on AKT as well as downstream molecules of the PI3K pathway, such as S6 ribosomal protein (S6RP) in breast, lung and colon cancer cell lines harboring a PI3KCA mutation. Additionally, blockade of AKT and molecules in the PI3K pathway leads to the inhibition of cell proliferation and cell transformation in cancer cells. Our data suggest that the inhibition of PDK-1 activity is sufficient to induce anti-tumor activity in cancer cells through the PI3K-PDK1-AKT axis, and that a potent and specific PDK-1 inhibitor could potentially be developed as a therapeutic agent against several cancer types.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4482.

Abstract 4479: PF-04691502, a potent and selective PI3K/mTOR dual inhibitor with antitumor activity

The PI3K pathway, which regulates cell growth, proliferation and survival, is activated in many types of human tumors by mutational activation of PI3Kα, loss of function of PTEN or activation of receptor tyrosine kinases. Inhibition of key signaling proteins in the pathway, such as PI3K, AKT and mTOR, therefore represents a high value targeting strategy for diverse cancers. PF-04691502 is a dual-specificity inhibitor of PI3K and mTOR which shows potent and selective activity in in vitro biochemical, cell and xenograft models.

In in vitro biochemical assays PF-04691502 inhibited recombinant PI3Kα, β, γ and δ isoforms with Ki's of 1.2-2.2 nM and recombinant mTOR with a Ki of 9.1 nM. PF-04691502 demonstrated a high degree of selectivity for inhibition of PI3K family kinases as shown by lack of activity against a panel of &gt;75 protein kinases, including the Class III PI3K hVps34. PF-04691502 also inhibited transformation of avian fibroblasts mediated by PI3K γ, δ, mutant PI3Kα E545K or membrane-localized AKT with IC50's of ∼100nM. In cell assays PF-04691502 inhibited PI3K/mTOR signaling in SKOV3 ovarian cancer cells with PI3Kα mutations and in U87MG glioblastoma cells with PTEN alteration, as indicated by reduced levels of phosphorylation of AKT(T308), AKT(S473) and S6 ribosomal proteins. Functional studies for anti-proliferative effects suggest PF-04691502 has broad efficacy across tumor types.

In SKOV3 and U87MG xenograft models PF-04691502 treatment resulted in dose-dependent tumor growth inhibition (TGI) with maximum TGI of ∼70% at the maximum tolerated dose of 10 mg/kg, by once daily oral dosing. Inhibition of AKT(S473) phosphorylation and S6RP(S235/236)/PRAS40(T246)/4EBP1(T37/46) phosphorylation were used as quantitative and qualitative pharmacodynamic (PD) endpoints, respectively; a clear pharmacokinetic (PK)/PD relationship was established in both models after multiple dose oral administration. In the U87MG xenograft model AKT(S473) phosphorylation was inhibited with an estimated EC50 of 5.7 nM (free plasma concentration) based on PK/PD modeling. The free plasma Area Under Curve was estimated to be 850 nM*hr for 70% TGI at 10mg/kg and was found to be similar in the SKOV3 model. The projected human efficacious dose of 10 mg once daily oral dosing provides Caverage steady state exposure of 22.4 nM (free plasma concentration) which is sufficient for 50-80% inhibition of pAKT S473, and corresponds to 74% TGI. Phase 1 clinical trials of PF-04691502 as a single agent are planned.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4479.

Abstract LB-302: Activity of PF-04691502, A dual PI3K/mTOR inhibitor in breast cancer cell lines and models discriminates between ER, PR and HER2 positive and negative segments

The PI3K pathway plays a role in key cellular functions that include regulation of cell growth, proliferation, survival, angiogenesis, and motility. PI3K/AKT pathway aberrations are common in cancer comprising PTEN loss of function through mutations, deletion, and methylation events and gain of function at the PIK3CA locus including mutations and amplification events resulting in deregulation of this pathway. Thus, pharmacological intervention of this pathway should impact cellular functions central to survival of cancer cells. PIK3CA mutations/amplification and PTEN loss of heterozygosity have been reported to occur in approximately 35-40% and 20% of breast cancer, respectively and suggest this patient population could benefit from treatment using a small molecule inhibitor that targets the PI3K pathway. PF-04691502 which is a potent inhibitor of all PI3K isoforms and mTOR (TORC1 and TORC2) presently in Phase 1 trials was evaluated for its growth inhibitory or cytoreductive activity over a panel of 30 breast cancer cell lines. PF-04691502 had robust &lt;300nM (IC50%) antiproliferative activity across the panel of breast cell lines. We observed a trend whereby cell lines that were hormone receptor (ER and/or PR) and HER2 negative (triple negative) were least sensitive and cell lines that were HER2(+) and/or hormone receptor positive (ER and/or PR) were more sensitive to PF-04691502. These studies were extended in vivo with PF-04691502 as a single agent and in combination in a HER2, ER and PR positive xenograft model with robust tumor growth inhibition in combination with docetaxel or with the pan-HER inhibitor, PF-00299804. In a xenograft that was triple negative, administration of single agent PF-4691502 at the MTD resulted in 60% tumor growth inhibition. In conclusion, our findings suggest that the ER/PR and/or HER2 positive breast cancer segment may particularly benefit from PI3K/mTOR inhibitor based treatment regimens.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-302.

Abstract 753: Novel, potent and selective small molecule inhibitors of 3-phosphoinositide-dependent kinase (PDK1)

The phosphatidylinositol 3-kinase (PI3K) signaling pathway plays a crucial role in cell growth, proliferation and survival. Genomic aberrations in the PI3K pathway, such as mutational activation of PI3Kα or loss-of-function of the tumor suppressor PTEN, have been closely linked to the development and progression of a wide range of cancers. Inhibition of the key targets in the pathway, PI3K, AKT, mTOR & PDK1, may provide an effective treatment of cancer. In an effort to discover compounds that inhibit PDK1, we have developed a series of 3-Carbonyl-4-Amino-Pyrrolopyrimidine (CAP) compounds that are selective and potent PDK1 inhibitors. Early screening led to a viable starting point, PF-03772304, (4-amino-7-isopropyl-7H-pyrrolo[2,3-d]pyrimidin-5-yl)-(6-methylamino-pyrazin-2-yl)-methanone, which has an IC50 of 94 nM for PDK1 and a ligand efficiency of 0.42. While potent, this lead was not selective against PI3K. Using structure-based drug design, this lead was modified to expand into the selectivity pocket of PDK1 (under the G-Loop), leading to the identification of a potent and pathway-selective compound, PF-05017255 ((4-Amino-7-isopropyl-7H-pyrrolo[2,3-d]pyrimidin-5-yl)-{6-[(3S,4R)-4-(4-fluoro-phenyl)-tetrahydro-furan-3-ylamino]-pyrazin-2-yl}-methanone). PF-05017255 has a Ki of 0.6 nM for PDK1 and is more than 400-fold selective against other PI3K pathway kinases: PI3Kα, AKT, S6K and mTOR. For even greater kinase selectivity, we sought to lower the clogP of our lead (clogP for PF-05017255 is 3.0) to reduce the contribution from the hydrophobic effect. These efforts led to PF-05168899 (1-{(2R,3R)-3-[6-(4-Amino-7-isopropyl-7H-pyrrolo[2,3-d]pyrimidine-5-carbonyl)-pyrazin-2-ylamino]-2-phenyl-pyrrolidin-1-yl}-ethanone) with a Ki of 0.4 nM for PDK1, a clogP of 2.1, and greater than 1000-fold selectivity against PI3Kα, AKT, S6K, mTOR, CDK2, CHK1 and PAK4. PF-05168899 also showed little inhibitory effect (&lt;50% at 1 uM) against 33 of 35 kinases in a broader panel, demonstrating significant inhibition only against CHK2 (94%) and AuroraB (54%). In addition, the most potent analogs (e.g. PF-05168889) inhibited the phosphorylation of AKT at the residue threonine 308 (IC50 40-200 nM) in a variety of cancer cell lines (e.g. H460, A549). The design, synthesis and SAR of this chemical series will be described.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 753.

Abstract 5779: The discovery of the potent and selective PI3K/mTOR dual inhibitor PF-04691502 through structure-based drug design

The phosphatidylinositol 3-kinase (PI3K) signaling pathway plays crucial roles in cell growth, proliferation and survival. Genomic aberrations in the PI3K pathway, such as mutational activation of PI3Kα or loss of function of tumor suppressor PTEN, have been closely linked to the development and progression of a wide range of cancers. Hence, inhibition of the key targets in the pathway, e.g. PI3K, AKT, mTOR, offers great potential for the treatment of cancer. In an effort to discover compounds that inhibit PI3Kα, a high throughput screen was carried out, and 4-methyl-pyrido-pyrimidine (MPP) derivatives were identified as potent and selective inhibitors of PI3Kα. For example, PF-00271897, 8-cyclopentyl-6-[3-(hydroxymethyl)phenyl]-4-methyl-2-(methylamino)pyrido[2,3-d]pyrimidin-7(8H)-one demonstrated PI3Kα Ki of 2.2 nM. Multiple crystal structures of inhibitors bound to PI3K gamma were determined to inform design and optimization of the ADMET properties of this lead series. Crystallographic studies with PI3K gamma protein indicated that the aminopyrimidine moiety forms two hydrogen bonds to the kinase backbone, and the aromatic moiety at the 6 position binds in a hydrophobic pocket. The X-ray structure suggested that the 4-methyl group on the MPP core structure conferred the excellent overall kinase selectivity to the series. The structure and SAR suggested optimization could come from keeping N-R group at 2 position very small and maintaining aromatic moiety at 6 position for hydrophobic interaction. Introduction of polar groups to the 8N side chains that are located in the ribose binding pocket increased both metabolic stability and solubility. Based on the overall properties, PF-04691502, 2-amino-8-[trans-4-(2-hydroxyethoxy)cyclohexyl]-6-(6-methoxypyridin-3-yl)-4-methylpyrido[2,3-d]pyrimidin-7(8H)-one, was selected as a clinical candidate. PF-04691502 demonstrated Ki's of 1.2-2.2 nM against PI3K α, β, γ and δ isoforms, and Ki of 9.1 nM against recombinant mTOR. PF-04691502 inhibited AKT phosphorylation at S473 in BT20 breast cancer line with IC50 of 12 nM. PF-04691502 is highly selective for inhibition of PI3K family kinases as shown by lack of activity against a panel of &gt;75 protein kinases, including the class III PI3K hVps34. In the in vivo rat PK studies, PF-04691502 demonstrated the following properties: Clearance = 5.2 ml/min/kg, Vdss = 1.4 L/kg, T1/2 = 3.1 h, F% = 63%. The design, synthesis, in vitro potency SAR, selectivity, ADMET of the MPP derivatives will be discussed. The crystal structure of PF-04691502 in PI3K gamma will also be presented.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5779.

2-Morpholino-4-oxo-4,5-dihydrothiophene-3-carbonitrile

The title compound, C₉H₁₀N₂O₂S, was obtained from the treatment of ethyl 4-cyano-3-hydr­oxy-5-morpholinothio­phene-2-carboxyl­ate with concentrated HCl. The mean plane of the essentially planar dihydro­thio­phene ring is almost orthogonal to the mirror plane of the N-morpholine substituent, making a dihedral angle of 87.2 (2)°.

Design and SAR of thienopyrimidine and thienopyridine inhibitors of VEGFR-2 kinase activity

Novel classes of thienopyrimidines and thienopyridines have been identified as potent inhibitors of VEGFR-2 kinase. The synthesis and SAR of these compounds is presented, along with successful efforts to diminish EGFR activity present in the lead series.

Pharmacological characterization of CP-547,632, a novel vascular endothelial growth factor receptor-2 tyrosine kinase inhibitor for cancer therapy

Signaling through vascular endothelial growth factor (VEGF) receptors (VEGFRs) is a key pathway initiating endothelial cell proliferation and migration resulting in angiogenesis, a requirement for human tumor growth and metastasis. Abrogation of signaling through VEGFR by a variety of approaches has been demonstrated to inhibit angiogenesis and tumor growth. Small molecule inhibitors of VEGFR tyrosine kinase have been shown to inhibit angiogenesis, inhibit tumor growth, and prevent metastases. Our goal was to discover and characterize an p.o. active VEGFR-2 small molecule inhibitor. A novel isothiazole, CP-547,632, was identified as a potent inhibitor of the VEGFR-2 and basic fibroblast growth factor (FGF) kinases (IC(50) = 11 and 9 nM, respectively). It is selective relative to epidermal growth factor receptor, platelet-derived growth factor beta, and other related TKs. It also inhibits VEGF-stimulated autophosphorylation of VEGFR-2 in a whole cell assay with an IC(50) value of 6 nM. After oral administration of CP-547,632 to mice bearing NIH3T3/H-ras tumors, VEGFR-2 phosphorylation in tumors was inhibited in a dose-dependent fashion (EC(50) = 590 ng/ml). These plasma concentrations correlated well with the observed concentrations of the compound necessary to inhibit VEGF-induced corneal angiogenesis in BALB/c mice. A sponge angiogenesis assay was used to directly compare the inhibitory activities of CP-547,632 against FGF receptor 2 or VEGFR-2; this compound potently inhibits both basic FGF and VEGF-induced angiogenesis in vivo. The antitumor efficacy of this agent was evaluated after once daily p.o. administration to athymic mice bearing human xenografts and resulted in as much as 85% tumor growth inhibition. CP-547,632 is a well-tolerated, orally-bioavailable inhibitor presently under clinical investigation for the treatment of human malignancies.

Correlation and Prediction of Phenytoin Protein Binding Using Standard Laboratory Parameters in Patients After Renal Transplantation

Renal transplant recipients provide a unique model for protein-binding studies in that patients experience hypoalbuminemia and renal dysfunction, both of which alter protein binding. The purposes of this investigation were to model the relationship between serum creatinine, blood urea nitrogen (BUN), albumin, and the unbound fraction of phenytoin (FU, as a percentage) in patients who had undergone renal transplant, and to determine the value of these measurements in predicting FU. Blood from 29 patients was collected at various time points after establishment of graft function. Sera were spiked with phenytoin to a concentration of 15 mg/L, and total/unbound phenytoin concentrations were determined. Correlations between FU and the biochemical indices of serum creatinine, BUN, and albumin were determined using multiple regression. The algorithm with the highest correlation at all times after the transplant became the method to predict future FU. This algorithm was applied prospectively in 23 samples from 14 other patients with variable renal function after transplant. Samples were analyzed as above and the corresponding biochemical indices of serum creatinine, BUN, and albumin were used to calculate FU values. Accuracy of the predictions was evaluated using prediction-error analysis. The best relationship between FU and the measured biochemical indices incorporated serum creatinine and albumin [y = 24.3 + 0.6(serum creatinine) - 3.9(albumin)] and served as the method for FU prediction. Prediction-error analysis resulted in a bias of -5.1% and a precision of 5.7%. This method failed to estimate FU with sufficient accuracy to permit clinical utility. The predicted value underestimated the measured value, and some other variable(s) must be affecting the binding even though serum creatinine and albumin are within or approaching the reference range. Consequently, estimating FU in patients with a history of uremia and hypoalbuminemia, based on measures of serum creatinine and albumin alone, should not be used.

Cefazolin dialytic clearance by high-efficiency and high-flux hemodialyzers

Cefazolin dialytic clearance has not been determined in patients undergoing hemodialysis with high-efficiency or high-flux dialyzers. The objective of this study is to determine the pharmacokinetics and dialytic clearance of cefazolin and develop dosing strategies in these patients. Twenty-five uninfected subjects undergoing chronic thrice-weekly hemodialysis were administered a single dose of intravenous cefazolin (15 mg/kg) after their standard hemodialysis session. Fifteen subjects underwent hemodialysis with high-efficiency hemodialyzers, and 10 subjects underwent hemodialysis with high-flux hemodialyzers. Blood and urine samples were collected serially over the interdialytic period, during the next intradialytic period, and immediately after the next hemodialysis session. Serum and urine concentrations of cefazolin were determined by high-performance liquid chromatography. Differential equations describing a two-compartment model were fit to the cefazolin serum concentration-time data over the study period, and pharmacokinetic parameters were determined. Mean dialytic clearance values for cefazolin were significantly greater in the high-flux group compared with the high-efficiency group (30.9 +/- 6.52 versus 18.0 +/- 6.26 mL/min, respectively; P: < 0.05). Cefazolin reduction ratios were significantly greater (0.62 +/- 0.08 versus 0.50 +/- 0.07; P: < 0.005) in the high-flux group compared with the high-efficiency group and correlated well with equilibrated urea reduction. The pharmacokinetic model developed from patient data was used to simulate cefazolin serum concentration data for high-efficiency and high-flux dialyzers. Cefazolin doses of 15 or 20 mg/kg after each hemodialysis session maintained adequate serum concentrations throughout a 2- or 3-day interdialytic period regardless of hemodialyzer type.

Trends in crime and the introduction of a needle exchange program

OBJECTIVES

This study sought to determine whether introduction of a needle exchange program would be associated with increased crime rates.

METHODS

Trends in arrests were compared in program and nonprogram areas before and after introduction of a needle exchange program in Baltimore. Trends were modeled and compared via Poisson regression.

RESULTS

No significant differences in arrest trends emerged. Over the study period, increases in category-specific arrests in program and nonprogram areas, respectively, were as follows: drug possession, 17.7% and 13.4%; economically motivated offenses, 0.0% and 20.7%; resistance to police authority, 0.0% and 5.3%; and violent offenses, 7.2% and 8.0%.

CONCLUSIONS

The lack of association of overall and type-specific arrest data with program implementation argues against the role of needle exchange programs in increasing crime rates.

Comparing new participants of a mobile versus a pharmacy-based needle exchange program

OBJECTIVE

To compare characteristics of first-time needle exchange participants who enrolled at a mobile van-based exchange site versus a fixed pharmacy-based exchange site, in an area where both types of needle exchange programs were available.

METHODS

Demographic and drug use data were collected on needle exchange program participants on enrollment. Participants were included if they were first-time participants at the Baltimore needle exchange program between December 1997 and March 1999, and if their first visit was at either one van-based site or at one of two pharmacy-based sites. Descriptive statistics and inferences were based on the type of needle exchange into which participants enrolled.

RESULTS

Among 286 first-time participants, 92% were African American, 28% were women, 11% were currently employed, 55% completed high school, and the median age was 40 years. In multivariate analyses, van-based enrollment was more common among frequent injectors (odds ratio [OR] = 2.0), but less common among African American participants (OR = 0.21).

CONCLUSIONS

Our data suggest that different venues for needle exchange program settings attract different types of drug injecting participants. This suggests that offering different venue types to reach participants with differing drug use patterns will be important to optimize risk reduction strategies.