Establishment of a t(11;19), KMT2A Rearranged B-ALL Cell Line for Preclinical Evaluation and Novel Therapeutics Development for Refractory Infant Leukemia

Leukemia, diagnosed in children less than 12 months of age, is a rare condition with an aggressive disease presentation and poor response to conventional chemotherapeutic agents. In addition, the unique vulnerability of the affected population does not always permit the use of markedly intense regimens with higher doses of cytotoxic agents. However, the unique biology of these leukemic cells also provides opportunities for the identification of effective and potentially well-tolerated targeted therapeutic strategies. In this report, we describe the establishment and characterization of a cell line from the blasts of an infant diagnosed with refractory B-cell acute lymphoblastic leukemia (ALL) carrying the characteristic histone lysine methyltransferase 2A (KMT2A) gene rearrangement. This cell line consists of rapidly proliferating clones of cells with chemosensitivity patterns previously described for KMT2A rearranged leukemia cells, including relative resistance to glucocorticoids and sensitivity to cytarabine. We also show effective targetability with menin inhibitors, indicating the activity of abnormal KMT2A-related pathways and the potential utility of this cell line in comprehensive drug library screens. Overall, our findings report the establishment and in vitro validation of a cell line for research into key aspects of infant leukemia biology and targeted therapeutics development.

Inhibition of p90 ribosomal S6 kinases disrupts melanoma cell growth and immune evasion

BACKGROUND

The mitogen-activated protein kinase (MAPK) signaling pathway is frequently hyperactivated in malignant melanoma and its inhibition has proved to be an efficient treatment option for cases harboring BRAFV600 mutations (BRAFMut). However, there is still a significant need for effective targeted therapies for patients with other melanoma subgroups characterized by constitutive MAPK activation, such as tumors with NRAS or NF-1 alterations (NRASMut, NF-1LOF), as well as for patients with MAPK pathway inhibitor-resistant BRAFMut melanomas, which commonly exhibit a reactivation of this pathway. p90 ribosomal S6 kinases (RSKs) represent central effectors of MAPK signaling, regulating cell cycle progression and survival.

METHODS

RSK activity and the functional effects of its inhibition by specific small molecule inhibitors were investigated in established melanoma cell lines and patient-derived short-term cultures from different MAPK pathway-hyperactivated genomic subgroups (NRASMut, BRAFMut, NF-1LOF). Real-time qPCR, immunoblots and flow cytometric cell surface staining were used to explore the molecular changes following RSK inhibition. The effect on melanoma cell growth was evaluated by various two- and three-dimensional in vitro assays as well as with melanoma xenograft mouse models. Co-cultures with gp100- or Melan-A-specific cytotoxic T cells were used to assess immunogenicity of melanoma cells and associated T-cell responses.

RESULTS

In line with elevated activity of the MAPK/RSK signaling axis, growth and survival of not only BRAFMut but also NRASMut and NF-1LOF melanoma cells were significantly impaired by RSK inhibitors. Intriguingly, RSK inhibition was particularly effective in three-dimensional growth settings with long-term chronic drug exposure and suppressed tumor cell growth of in vivo melanoma models. Additionally, our study revealed that RSK inhibition simultaneously promoted differentiation and immunogenicity of the tumor cells leading to enhanced T-cell activation and melanoma cell killing.

CONCLUSIONS

Collectively, RSK inhibitors exhibited both multi-layered anti-tumor efficacy and broad applicability across different genomic melanoma subgroups. RSK inhibition may therefore represent a promising novel therapeutic strategy for malignant melanoma with hyperactivated MAPK signaling.

Abstract P4-01-16: High levels of RSK2 in breast cancer patients is associated with longer PFS in patients treated with PMD-026, a first in class RSK inhibitor

Background: Breast cancer (BC) is the most common malignancy in women and metastatic triple negative breast cancer (mTNBC) remains one of the most difficult to treat cancers with few targeted treatment options. RSK is recognized as a critical signaling component in the MAPK/PDK-1 pathways, is an important driver for BC and a signature of poor prognosis. PMD-026 is the first RSK inhibitor to enter clinical trials and is being developed alongside an immunohistochemistry (IHC) companion diagnostic to select patients with increased activated RSK2 in tumor tissue. A Phase 1/1b trial of PMD-026 in patients with metastatic breast cancer (mBC) or metastatic triple negative breast cancer (mTNBC) established safety at a dose of 200 mg Q12h. Efficacy signals in patients with heavily pretreated mBC/mTNBC are explored in this analysis along with evaluation of the effect of food (FE) on systemic exposure to treatment. Methods: PMD-026 was administered to 41 patients as a single agent in this phase 1/1b open-label study, with 30 patients evaluable for efficacy. Exploratory objectives were to identify subgroups of patients who may optimally benefit from PMD-026. Subgroup analysis of patients included 1) comparing BC patients who received ≤5 vs >5 prior therapies; 2) comparing TNBC patients (de novo vs secondary subtypes)1, and 3) comparing patients with low RSK2 H-scores (< 180) vs high (≥180). In addition, PMD 026 PK was evaluated at the 200 mg Q12h dose and a FE sub-study enrolled 12 patients administered a single 200 mg dose. Results: PMD-026 monotherapy was generally well-tolerated in the 41 mBC patients who were enrolled and treated. Kaplan-Meier PFS analysis of 30 evaluable BC patients who were dosed with PMD-026 showed that patients with less prior therapy (≤5) did significantly better (HR, 0.19; 95% CI [0.06–0.52], p=0.0014) than those with > 5 prior therapies. Subgroup analysis of PFS in those with TNBC demonstrated that de novo TNBC (n=17) had longer time on treatment with PMD-026 compared with secondary TNBC (n=9) (HR, 0.31; 95% CI [0.10-0.99], p=0.0476). In those with de novo TNBC with ≤5 prior therapies, a high RSK2 H-score was associated with significantly longer PFS at the RP2D (4.2 vs 1.3 months, HR, 0.17; 95% CI [0.03-0.80], p=0.0254) than patients with a low RSK2 H-score. In patients with CDK4/6 resistant HR+ BC (n=3), PFS was 5.2 (RSK2 high) vs 1.3 months (RSK2 low). Stable disease was observed in 53% (9/17) of patients with de novo TNBC and in 67% (6/9) of de novo TNBC patients with high RSK2. Tumor necrosis or target lesion reduction (< 30%) was observed in 17% of patients (5/30), all of whom had high RSK2 expression. In the FE sub-study, increased interpatient variability in PMD-026 Cmax and Tmax but not AUC, was observed when administered with food, favored dosing in a fasted state, which is consistent with the pH dependent solubility of PMD-026. Notably, all FE patients (12/12) achieved the target concentration of 1µM (IC90 in preclinical studies) within 4 hours when PMD-026 was taken without food. At the RP2D, PMD-026 taken without food showed relatively consistent exposure among patients over 24 hr timeframe. Conclusions: These findings demonstrate that in patients treated with PMD-026 who had received < 5 prior treatment regimens, had de novo TNBC or CDK4/6 refractory HR+ disease and had high RSK2 scores had longer PFS. Overall, PMD-026 is a well-tolerated, orally available RSK2 inhibitor that will be evaluated further for efficacy in TNBC and CDK4/6i refractory HR+ mBC, in a trial that will prospectively enroll patients based on RSK2 activation as defined by the RSK2 IHC H-scores. Clinical trial information: NCT04115306. 1 Patients diagnosed and treated for TNBC from their initial diagnosis (de novo TNBC) vs patients previously treated for hormone receptor positive (HR+) or human epidermal growth factor 2 receptor positive (HER2+) BC, but became HR or HER2 negative (secondary TNBC)

Citation Format: Judy S. Wang, Muralidhar Beeram, Pavani Chalasani, Lida Mina, Rebecca A. Shatsky, Sara Hurvitz, Meghna S. Trivedi, Robert Wesolowski, Hyo S. Han, Amita Patnaik, Shakeela Bahadur, My-my Huynh, Aarthi Jayanthan, Gerrit Los, Sandra E. Dunn, Andrew Dorr. High levels of RSK2 in breast cancer patients is associated with longer PFS in patients treated with PMD-026, a first in class RSK inhibitor [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-01-16.

Abstract 5378: PMD-026, a first in class oral RSK inhibitor, demonstrates activity against hormone receptor positive breast cancer with acquired CDK4/6 inhibitor resistance

PMD-026 is a first in class, reversible, oral small molecule inhibitor of p90 ribosomal S6 kinase (RSK), a kinase family activated by the MAPK, PDK-1 and PI3K pathways, which regulate substrates involved in cancer cell proliferation and drug resistance. Specifically, both MAPK and PI3K pathways are implicated in resistance to standard of care (SOC) CDK4/6 inhibitors in hormone receptor positive (HR+) breast cancer (BC). RSK2 is expressed in 80% of HR+ BC, therefore we hypothesized that disrupting these critical pathways with PMD-026 could overcome resistance to CDK4/6 inhibitors through cell growth inhibition, apoptosis induction and downstream target inhibition alone and in combination with SOC SERD fulvestrant. In preclinical studies and a Phase I/Ib clinical trial in metastatic BC, PMD-026 demonstrated a good safety profile, making it an ideal candidate for the CDK4/6 inhibitor resistant HR+ BC population. Here in we demonstrate that PMD-026 inhibits growth of HR+ BC cell lines MCF-7 and T47D in vitro at IC50 values ranging from 3.7 to 10.1 µM, resulting in dephosphorylation of the RSK downstream effector YB-1 and induction of apoptosis through PARP cleavage. Comparatively, the MAPK pathway remains active in the presence of fulvestrant, as indicated by the expression of pYB-1. We determined that PMD-026 synergizes with fulvestrant in the MCF-7 model in vitro with a combined drug index (CDI) of 0.73 to 0.92. The next step was to determine if this combination is effective in cells resistant to CDK4/6 inhibitors. For that purpose, MCF-7 and T47D cells were treated with increasing concentrations of the CDK4/6 inhibitor palbociclib (Ibrance) up to 2 µM for several months. The generated Ibrance resistant cell lines (MCF-7-IBR, T47D-IBR) are insensitive to Ibrance up to 2 µM, but their parental counterparts have IC50 values ranging from 0.05 to 0.15 µM. In addition, cross-resistance to an additional CDK4/6 inhibitor, abemaciclib (Verzenio), was observed in the resistant cell lines, but the parental lines remained sensitive at IC50 values ranging from 0.15 to 0.25 µM. Most importantly, both parental and resistant cell lines remained sensitive to PMD-026 at similar IC50 values, indicating that CDK4/6 inhibitor resistance does not affect PMD-026 sensitivity. PMD-026 was then tested in combination with fulvestrant in anchorage independent conditions in the MCF-7-IBR cells. It was determined that this combination was highly synergistic with CDI ranging from 0.55 to 0.79. Together, these data support the application of PMD-026 and fulvestrant as a novel method to stop the growth of HR+ BC with acquired resistance to CDK4/6 inhibitors. Additionally, given that the MAPK pathway is upregulated in response to PI3K inhibition, the PMD-026 and fulvestrant combination is a potential alternative treatment for HR+ BC patients.

Citation Format: Aarthi Jayanthan, Lambert Yue, My-my Huynh, Gerrit Los, Sandra E. Dunn. PMD-026, a first in class oral RSK inhibitor, demonstrates activity against hormone receptor positive breast cancer with acquired CDK4/6 inhibitor resistance [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 5378.

Abstract P5-17-12: First-in-human expansion study of oral PMD-026 in metastatic triple negative breast cancer patients

Background: Durably effective therapeutic options remain elusive for metastatic triple negative breast cancer (mTNBC) patients. RSK is a novel target kinase for mTNBC, given its integral role in the MAPK/PDK-1 pathways. PMD-026, uniquely developed for TNBC, is a first-in-class, potent, oral RSK inhibitor that constitutes a promising avenue of treatment for mTNBC. PMD-026 demonstrated a favorable safety profile and initial signs of clinical benefit in metastatic breast cancer patients in Phase I. The current expansion is investigating PMD-026 in mTNBC patients whose disease has progressed on standard therapy. Methods: This open-label study evaluates the safety and efficacy of single agent PMD-026 in mTNBC patients. Target accrual for this study is a minimum of 20 mTNBC patients dosed at 200 mg q 12 hours who have measurable disease. A food effect (FE) sub-study is enrolling a total of 12 patients with a two-arm crossover design. Exploratory biomarker analysis of tumor tissue is being assessed for activated RSK2 levels. Exploratory objectives are to understand TNBC heterogeneity, with a view to identify patients who may benefit from PMD-026 optimally. Results: Based on the trial results thus far from 25 patients, PMD-026 continues to be well-tolerated with no G4 treatment-related adverse events. The noted toxicities include low incidence of elevated ALT/AST, rash, colitis or low-grade nausea. There has been no hair loss, myelosuppression or peripheral neuropathy. Given that TNBC is such a heterogeneous disease, we sought to identify subsets of patients with extensive prior therapy (≥ 5 lines) who may benefit from PMD-026 as a monotherapy. Based on ongoing analyses in the Phase I and Ib, patients diagnosed with TNBC at their initial diagnosis (de novo TNBC) stayed on study 3-4 times longer than patients who were initially treated for HR+ or HER2+ breast cancer but lost HR or HER2 expression to become TNBC (secondary TNBC). In addition, de novo TNBC patients treated at the recommended phase II dose (RP2D) of 200 mg BID with an H score for RSK2 ≥ 180 had a median progression free survival (PFS) of 3.3 months (n=3). In contrast, patients with an H score < 180 had a median PFS of 0 months (n=3). Furthermore, the PFS of 3.3 months on PMD-026 is longer than the PFS of 1.7 months in a similar population of TNBC patients on chemotherapy, where the average number of prior treatments was 3a. . Conclusions: Updated safety, clinical activity, PK, and biomarker analyses will be presented. Clinical trial information: NCT04115306.aBardia et al, N Engl J Med 2021; 384:1529-154

Citation Format: Muralidhar Beeram, Judy S. Wang, Lida A. Mina, Pavani Chalasani, Rebecca A. Shatsky, Robert Wesolowski, Sara A. Hurvitz, Meghna S. Trivedi, Hyo S. Han, Amita Patnaik, My-my Huynh, Aarthi Jayanthan, Mary Rose Pambid, Lambert Yue, Gerrit Los, Sandra E. Dunn, Andrew Dorr. First-in-human expansion study of oral PMD-026 in metastatic triple negative breast cancer patients [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-17-12.

The T-type Calcium Channel Cav3.1 in Y79 Retinoblastoma Cells is Regulated by the Epidermal Growth Factor Receptor via the MAPK Signaling Pathway

PURPOSE

Retinoblastoma is the most frequent intraocular cancer in children. It is also one of the most common causes for enucleation and carries a significant morbidity rate in affected individuals. Hence, studies on its pathophysiological and growth regulatory mechanisms are urgently needed to identify more effective novel therapeutics.

METHODS

Using the Y79 retinoblastoma cell line, we investigated the electrophysiological and functional activities of the T-type voltage-gated calcium channel Cav3.1, that is constitutively expressed in these cells. We also analyzed the Akt and MAPK signaling pathways downstream of the epidermal growth factor receptor (EGFR) to understand the mechanism responsible for the inhibition of Cav3.1.

RESULTS

We demonstrate that the EGFR inhibitor Afatinib significantly reduced cell viability and Cav3.1 mRNA expression and electrophysiological activity. At low concentrations (1 µM), Afatinib reduced the amplitude of Cav3.1 current density, whereas at a high concentration (10 µM), it completely abolished the voltage-gated calcium current. Our results show that inhibition of the MAPK pathway by a specific inhibitor VX-11e affected the Cav3.1 current in a dose-dependent manner. VX-11e (50 nM-1 µM) treatment reduced Cav3.1 current densities in Y79 cells, with complete abolishment of Cav3.1 current at higher concentrations (5 µM). We also demonstrate that the specific inhibition of the Akt kinase (using MK-2206) had no effect on the Cav3.1 currents.

CONCLUSION

Our study provides a functional relationship between the MAPK pathway and EGFR signaling and indicates that the MAPK signaling pathway mediates the control of Cav3.1 by EGFR in retinoblastoma.

Abstract 1038: PMD-026, a first in class oral RSK inhibitor, demonstrates synergy when combined with standard of care in breast cancer tumor models

PMD-026 is a first in class, reversible, oral small molecule inhibitor of p90 ribosomal S6 kinase (RSK), a kinase family activated by the MAPK and PDK-1 pathways, which regulate substrates involved in cancer cell proliferation and drug resistance. Specifically, RSK2 has been identified as a major driver in breast cancer (BC). In preclinical studies and a Phase I clinical trial in metastatic BC, PMD-026 demonstrated a good safety profile, making it an attractive candidate for combinations with standard of care therapies. Here in we show that RSK2 is activated in 87% of triple negative breast cancer (TNBC), as well as in other subtypes (80% of ER+/PR+ and 81% of HER2+). In a screen of 28 BC cell lines across a broad mutational spectrum, PMD-026 induced apoptosis in most of the models. Next, to determine the role of PMD-026 in the context of conventional BC treatment, we combined it with standard of care chemotherapies fulvestrant, paclitaxel, or doxorubicin, in vitro and in vivo. PMD-026 synergized with fulvestrant, with combination drug index (CDI) values ranging from 0.73 - 0.92 in the HR+ cell line MCF-7. Likewise, PMD-026 synergized with paclitaxel and doxorubicin in TNBC cell lines with CDI values ranging from 0.40 - 0.84 and 0.58 - 0.92 for paclitaxel and doxorubicin, respectively. Consistent with in vitro screening data, PMD-026 synergized with paclitaxel in vivo in the SUM149PT TNBC xenograft model after 38 days of treatment. The combination inhibited tumor growth by 66% (P < 0.0001), whereas paclitaxel and PMD-026 as single agents inhibited tumor growth by 22% (P = 0.3051) and 41% (P = 0.0041), respectively. The synergy of this combination was further supported by a CDI value of 0.75. Treatment among the groups was well tolerated with no changes in body weight observed. Similarly, the combination of PMD-026 and paclitaxel was synergistic in SUM149PT(48 days, CDI: 0.78) and MDA-MB-231 (28 days, CDI: 0.66) xenografts and additive in a PDx model of metastatic TNBC (18 days, CDI: 0.96), resulting in improved survival. To address the safety of combining PMD-026 and paclitaxel, drug-drug interactions (DDI) were assessed. In cytochrome P450-mediated in vitro metabolism assays, PMD-026 showed weak inhibition of Cyp2C8 and Cyp3A4, the main enzymes responsible for paclitaxel metabolism. To understand whether PMD-026 might alter the metabolism of paclitaxel, this potential DDI was assessed in vivo. Pharmacokinetic analysis of PMD-026 (7 days repeat dosing) combined with paclitaxel (8 mg/kg IV dose Day 1 and Day 7) was evaluated in CD-1 mice, however, PMD-026 did not change the absorption, distribution, or blood levels of paclitaxel. Together, these data support adding PMD-026 to standard of care therapies in breast cancer, as they have demonstrated synergy. In particular, PMD-026 is synergistic with paclitaxel in multiple TNBC models, in which it improves efficacy without added toxicity.

Citation Format: Aarthi Jayanthan, My-my Huynh, Jangsoon Lee, Gerrit Los, Lambert Yue, Mary Rose Pambid, Naoto T. Ueno, Sandra E. Dunn. PMD-026, a first in class oral RSK inhibitor, demonstrates synergy when combined with standard of care in breast cancer tumor 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 1038.

First-in-human phase 1/1b expansion of PMD-026, an oral RSK inhibitor, in patients with metastatic triple-negative breast cancer

e13043

Background: P90 ribosomal S6 kinase (RSK) is an actionable molecular target against metastatic triple negative breast cancer (mTNBC). RSK is a major convergence point in the integral TNBC signaling pathways, MAPK and PDK-1. PMD-026 is a first-in-class oral RSK inhibitor with high selectivity. The dose escalation portion of this study established the RP2D of PMD-026 as 200 mg Q12. PMD-026 demonstrated good plasma exposure following oral dosing, with a T1/2 of ̃ 6 h (range 4-8 h), and achieved the targeted preclinical efficacious concentrations using a Q12h dosing schedule. PMD-026 also demonstrated a tolerable safety profile and initial signs of efficacy in patients with metastatic breast cancer. The intensity of RSK2 activation ranged from an H Score of 110-268 based on a CLIA companion immunohistochemistry assay. We present initial data from the expansion cohort. Methods: The primary aim of this single-arm, open-label, first-in-human phase 1/1b study is to evaluate the safety of single agent PMD-026 in patients with mTNBC. Secondary endpoints are clinical activity, pharmacokinetics, and correlative biomarker expression on tumor specimens. Patients are dosed at 200 mg twice daily in 21-day cycles. Eligible patients have measurable disease as per RECIST v1.1 and have had disease progression on or after standard of care treatment. Tumor tissue is assessed to retrospectively correlate RSK2 activity by immunohistochemistry (IHC) with clinical outcomes. Pharmacokinetics are assessed along with a food effect (sub-study with n=12). In addition, a pharmacodynamic marker, YB-1 phosphorylation, is being explored in peripheral blood mononuclear cells before and during treatment. Results: As of February 16, 2021, 7 patients with mTNBC (median age 62 years, range 33-74) have been enrolled in the phase 1b Expansion (median of 7 prior lines of therapy). Notable prior therapies in the phase 1b include sacituzumab govitecan (n=4) and atezolizumab/nab-paclitaxel (n=1). Patients in escalation and expansion treated with the RP2D had median progression free survival of 30 vs 99 days for low vs high RSK2 expression, respectively. This cut-off will be further evaluated in the expansion phase of the study. Conclusions: Updated safety, clinical activity, pharmacokinetic, and biomarker analyses will be presented. Target accrual for phase 1b Expansion is a minimum of 20 patients with mTNBC. Clinical trial information: NCT04115306.

Cytotoxicity and Target Modulation in Pediatric Solid Tumors by the Proteasome Inhibitor Carfilzomib

BACKGROUND

Most children with recurrent metastatic solid tumors have high mortality rates. Recent studies have shown that proteasome inhibition leads to effective tumor killing in cells that have acquired treatment resistance and metastatic properties.

OBJECTIVE

The purpose of this study was to test the potential of Carfilzomib (CFZ), a proteasome inhibitor, in refractory pediatric solid tumors, which is currently unknown.

METHODS

A panel of pediatric solid tumor cell lines, including neuroblastoma, Ewing's sarcoma, osteosarcoma, rhabdomyosarcoma, and atypical teratoid rhabdoid tumor (ATRT), was used to evaluate the cytotoxic and proteasomal inhibitory effects of CFZ. A drug scheduling experiment was performed to determine the optimal dose and time to obtain effective cell killing. Combination studies of CFZ with chemotherapeutic drugs of different classes were performed to determine the extent of synergy.

RESULTS

CFZ showed effective cytotoxicity against all cell lines tested (mean IC50 = 7nM, range = 1-20nM) and activity in a fluorophore-tagged cell-based proteasome assay. Drug scheduling experiments showed that the minimum exposure of 4-8 hours/day is needed for effective cumulative killing. CFZ, when combined with chemotherapeutic drugs of different classes, synergistically enhanced the extent of cell death.

CONCLUSIONS

CFZ showed cytotoxic activity against all the solid pediatric cancer cell lines tested. This study provides initial in vitro data on the potential of CFZ to treat pediatric solid tumors and supports further investigations into the components of drug scheduling, biological correlates, and drug combinations for future early phase clinical trials in children.

Abstract PS11-33: A first-in-human Phase 1/1b multicenter, open-label dose escalation study to assess safety and tolerability of PMD-026, a first-in-class oral RSK inhibitor, in metastatic breast cancer patients

Background: Metastatic breast cancer (mBC) remains an aggressive disease with limited durable treatment options; the worst prognosis among the breast cancer subtypes is typically seen in metastatic triple-negative breast cancer (mTNBC). Given that unmet need, we sought to identify an actionable molecular target to combat mTNBC. Promising preclinical activity identified p90 ribosomal s6 kinase 2 (RSK2) as a key kinase in mTNBC. PMD-026 is a potent, oral, small molecular RSK inhibitor with high selectivity for the RSK2 isoform. RSK is a major convergence point in the important MAPK and PDK-1 signaling pathways, which drive TNBC cell survival, proliferation, and drug resistance.

Methods: The primary aim of this single-arm, open-label, first-in-human, phase 1/1b study (NCT04115306) is to evaluate the safety of single agent PMD-026 in patients with mBC. Secondary endpoints are clinical activity, pharmacokinetics (PK) and correlative biomarker expression on tumor specimens. Patients are dosed orally once or twice daily in 21-day cycles with measures to adapt the dosing schedule based on the PK data, as needed. In dose escalation, patients must have mBC with evaluable or measurable disease by RECIST v1.1. In dose expansion, patients must have mTNBC with measurable disease by RECIST v1.1. Patients must have progressed on or after standard of care therapy. Tumor tissue is required to retrospectively correlate RSK2 activity with clinical outcomes via immunohistochemistry using a CAP/CLIA certified companion diagnostic (CDx).

Results: Twelve mBC patients (ER+ mBC n=5, mTNBC n=7) who have failed standard chemotherapy as well as targeted therapies such as CDK4/6 inhibitors and immunotherapies have been enrolled to date. Patients have been treated in escalating cohorts of 25, 50, 100, 200, 400 (200 q12) or 600 mg (300 q12) of PMD-026 administered orally daily. At 400 mg the dose schedule was changed from daily to q12 hrs based on PK results to optimize drug exposure over a 24-hr timeframe in patients. The PK of PMD-026 showed linear exposure and a high volume of distribution. The AUC was ~9100 hr*ng/ml on Day 1 when PMD-026 was dosed at 200 mg qd demonstrating high exposure. In addition, when dosed at 200 mg q12 hrs, PMD-026 serum levels approached the preclinically established desired level of 1 µM over 24 hrs. In the 200 mg q12 hrs cohort, adverse events consisted of G2 GERD (n=1) and G2 neutropenia (n=1). Initial signs of activity were observed as CT-identified necrosis in a neck node metastasis (n=1) and transient decrease in CA 27-29 (n=1). While the 200 mg q12 hrs dose was generally well-tolerated, there were 2 dose limiting toxicities at 300 mg q12 hrs including syncope (n = 1) and vomiting with dehydration leading to reversible acute kidney injury (n = 1). To further understand the patient population, RSK2 activation was assessed in tumor samples from all patients. RSK2 was activated in all of the tumors and the H-Score ranged from 110 to 198 using the CDx platform.

Conclusions: Preliminary evidence indicates that PMD-026 is well-tolerated at dose levels up to 200 mg q12 with initial signs of activity; pharmacokinetics showed good linear exposure. Updated safety, clinical activity, pharmacokinetic and biomarker analyses will be presented; target accrual for Phase 1b Expansion is approximately 20 mTNBC patients. (clinicaltrials.gov NCT04115306).

Citation Format: Murali Beeram, Judy S. Wang, Pavani Chalasani, Lida Mina, Amita Patnaik, Mary Rose Pambid, Aarthi Jayanthan, My-my Huynh, Gerrit Los, Sandra E. Dunn, F. Andrew Dorr. A first-in-human Phase 1/1b multicenter, open-label dose escalation study to assess safety and tolerability of PMD-026, a first-in-class oral RSK inhibitor, in metastatic breast cancer patients [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS11-33.

Human SNF5 arming of double-deleted vaccinia virus shows oncolytic and cytostatic activity against central nervous system atypical teratoid/rhabdoid tumor cells

Central nervous system (CNS) atypical teratoid/rhabdoid tumor (AT/RT) is a rare, aggressive tumor that most often affects very young children. The common decisive molecular defect in AT/RT has been shown to be a single genetic alteration, i.e., the loss of hSNF5 gene that encodes for a subunit of the SWI/SNF complex that modulates chromatin remodeling activities. As a result, AT/RT cells display unregulated cell proliferation due to the dysfunction of an important epigenetic control. We have previously demonstrated the preclinical efficacy of the oncolytic double-deleted vaccinia virus (VVDD) against AT/RT. Here we report the establishment of a modified VVDD engineered to express wild type hSNF5 gene. We show that this reconstructed vaccinia virus retains comparable infectivity and in vitro cytotoxicity of the parent strain. However, in addition, hSNF5-arming of VVDD results in a decreased cell cycle S phase population and down-regulation of cyclin D1. These findings suggest that hSNF5-arming of VVDD may increase the efficacy in the treatment of AT/RT and validates, as a proof-of-concept, an experimental approach to enhance the effective use of novel modified oncolytic viruses in the treatment of tumors with loss of a tumor suppressor gene function.

First-in-human phase I/Ib multicenter, open-label dose escalation study to assess safety and tolerability of PMD-026 in patients with metastatic breast cancer with expansion in metastatic triple negative breast cancer

TPS1110

Background: Metastatic triple negative breast cancer (mTNBC) has a poor prognosis with limited durable treatment options. RSK (P90 ribosomal S6 kinase) is a signaling protein at the convergence point of PDK-1 and MAPK signaling pathways. RSK1-3 phosphorylates transcription factors, including Y-box binding protein-1 (YB-1), thereby inducing drug resistance and cancer growth genes. Phosphorylated YB-1 is involved in tumor cell survival, proliferation, and drug resistance. In human breast tumor samples, RSK2 protein is expressed across all breast cancer subtypes (TNBC, ER+ and HER2+) and is associated with poor overall survival. Expression of RSK2 is found in approximately 87% of mTNBC tumors and of those tumors approximately 41% have very high expression of RSK2. PMD-026 is a potent, oral, small molecule RSK inhibitor with high selectivity for RSK2. Preclinical in vivo studies have demonstrated activity both as a single agent and in combination with standard of care therapies. Further, a CAP/CLIA certified IHC method has been developed with Roche to determine tumor expression of RSK2. Methods: This single-arm, open-label, first-in-human, phase I/Ib study evaluates the safety and efficacy of single agent PMD-026 in patients with metastatic breast cancer for whom standard therapies are no longer effective. During dose escalation, the study utilizes an accelerated titration design with single patient cohorts until the occurrence of DLT or Grade 2+ toxicity; then reverts to 3+3 design to define the maximally tolerated dose (MTD) and recommended phase II dose (RP2D). The dose expansion portion will enroll approximately 20 patients with mTNBC. Patients are dosed orally once daily in 21-day cycles with measures to adapt the dosing schedule based on the pharmacokinetic (PK) data, as needed. Tumor tissue is required for all enrolled patients; RSK2 expression will be retrospectively correlated with clinical outcomes. The primary objectives are to determine safety and tolerability of PMD-026, determine the MTD, define a RP2D, and assess anti-tumor activity of PMD-026 in patients with TNBC. Secondary objectives are to evaluate PK, time to response, mTNBC subtyping using NanoString, and duration of response of PMD-026. To date, cohorts 1 and 2 have been completed without DLT. Enrollment to cohort 3 began in January 2020. Clinical trial information: NCT04115306 .

Targeted Polo-like Kinase Inhibition Combined With Aurora Kinase Inhibition in Pediatric Acute Leukemia Cells

BACKGROUND

Recent studies have shown that cell cycle events are tightly controlled by complex and shared activities of a select group of kinases. Among these, polo-like kinases (Plks) are regulatory mitotic proteins that are overexpressed in several types of cancer and are associated with poor prognosis.

MATERIALS AND METHODS

We have evaluated, in preclinical in vitro studies, the activity of a panel of Plk inhibitors against cell lines derived from refractory pediatric leukemia, as well as primary leukemia cells, in culture. Through in vitro growth inhibition studies, Western blot analysis for the expression and activation of key regulators of cell growth and survival and gene silencing studies, we specifically examined the ability of these agents to induce cytotoxicity through the activation of apoptosis and their capacity to interact and modulate the expression and phosphorylation of Aurora kinases.

RESULTS

Our findings show that the various Plk-1 inhibitors in development show potential utility for the treatment of pediatric leukemia and exhibit a wide range of phosphorylation and target modulatory capabilities. Finally, we provide evidence for a complex interregulatory relationship between Plk-1 and Aurora kinases enabling the identification of synergy and biologic correlates of drug combinations targeting the 2 distinct enzyme systems.

DISCUSSION

This information provide the rationale for the evaluation of Plk-1 as an effective target for therapeutics in refractory pediatric leukemia and indicate compensatory activities between Plk-1 and Aurora kinases, providing insight into some of the complex mechanisms involved in the process of cell division.

In Vitro Sensitivity Profiling of Neuroblastoma Cells Against A Comprehensive Small Molecule Kinase Inhibitor Library to Identify Agents for Future Therapeutic Studies

BACKGROUND

Neuroblastoma (NB) constitutes about 8% of all childhood tumors, yet accounts for more than 15% of deaths, with an unacceptable overall survival rate. These rates are despite the current multimodal therapeutic approaches involving surgery, radiation, chemotherapy and myeloablation with hematopoietic stem cell rescue. Hence, efforts have intensified to identify new targets and novel therapeutic approaches to improve cure rates in these children. Numerous new agents for adult malignancies are developed and evaluated for cancer each year, providing an invaluable resource, with the added advantage of available pharmacologic and toxicity data for consideration.

METHODS

To identify potential therapeutic targets, we screened a small molecule library of 151 small kinase inhibitors against NB cell lines. Based on our initial screening data, we further examined the potential of Bcr-Abl targeting small molecule inhibitors to affect the growth and survival of NB cells.

RESULTS

There is diverse activity among the currently available Bcr-Abl inhibitors, possibly reflecting the molecular heterogeneity and off-target activity in each combination. In depth analyses of ponatinib, an oral multi-target kinase inhibitor and effective agent in the treatment of refractory Philadelphia chromosome (Ph) positive leukemia, show growth inhibition at sub-micromolar concentrations. In addition, we also identified the potential of this agent to interfere with insulin-like growth factor-1 receptor (IGF-1R) signaling pathways and Src activity, inhibit cell migration and induce apoptosis.

CONCLUSION

Our findings provide initial data on ponatinib's potential to target key growth regulatory pathways and provide the rationale for further studies and evaluation in future early phase clinical trials for the treatment of refractory NB.

Druggable targets in pediatric neurocutaneous melanocytosis: Molecular and drug sensitivity studies in xenograft and ex vivo tumor cell culture to identify agents for therapy

BACKGROUND

Neurocutaneous melanocytosis (NCM) is a rare congenital disorder that presents with pigmented cell lesions of the brain or leptomeninges in children with large or multiple congenital melanocytic nevi. Although the exact pathological processes involved are currently unclear, NCM appears to arise from an abnormal development of melanoblasts or melanocyte precursors. Currently, it has an extremely poor prognosis due to rapid disease progression and lack of effective treatment modalities.

METHODS

In this study, we report on an experimental approach to examining NCM cells by establishing subcutaneous tumors in nude mice, which can be further expanded for conducting molecular and drug sensitivity experiments.

RESULTS

Analysis of the NRAS gene-coding sequences of an established NCM cell line (YP-MEL) and NCM patient cells revealed heterogeneity in NRAS Q61K that activated mutation and possibly consequential differential sensitivity to MEK inhibition. Gene expression studies were performed to compare the molecular profiles of NCM cells with normal skin fibroblasts. In vitro cytotoxicity screens of libraries of targeted small-molecule inhibitors revealed prospective agents for further evaluation.

CONCLUSIONS

Our studies provide an experimental platform for the generation of NCM cells for preclinical studies and the production of molecular and in vitro data with which to identify druggable targets for the treatment.

Aurora kinases as druggable targets in pediatric leukemia: heterogeneity in target modulation activities and cytotoxicity by diverse novel therapeutic agents

Leukemia is the most common pediatric malignancy, constituting more than 30% of all childhood cancers. Although cure rates have improved greatly, approximately one in five children relapse and poor survival rates post relapse remain a challenge. Given this, more effective and innovative therapeutic strategies are needed in order to improve prognosis. Aurora kinases, a family of serine/threonine kinases essential for the regulation of several mitotic processes, have been identified as potential targets for cancer therapeutics. Elevated expression of Aurora kinases has been demonstrated in several malignancies and is associated with aberrant mitotic activity, aneuploidy and alterations in chromosomal structure and genome instability. Based on this rationale, a number of small molecule inhibitors have been formulated and advanced to human studies in the recent past. A comparative analysis of these agents in cytotoxicity and target modulation analyses against a panel of leukemia cells provides novel insights into the unique mechanisms and codependent activity pathways involved in targeting Aurora kinases, constituting a distinctive preclinical experimental framework to identify appropriate agents and combinations in future clinical studies.

Abstract C175: Effective targeting of PI3K/mTOR pathways with GDC-0980 in FLT-3 mutant pediatric leukemia

Introduction: FLT-3 is a receptor tyrosine kinase that is abnormally activated in a significant percentage of pediatric acute myeloid leukemia (AML) and infant ALL. Previous studies have shown that FLT-3 aberration with deregulated PI3K/Akt and mTOR signaling pathways have significantly poor outcome. GDC-0980 is an orally bioavailable inhibitor with selective and potent activity against PI3K and mTOR kinases with the capacity to target two crucial pathway nodes to produce the strongest accessible inhibition of signaling by these mechanisms. Methods: A panel of pediatric cell lines with FLT-3 mutations (N=3), ITD (n=3), Wt (n=2) and primary specimens (n=4) were evaluated. Cells were cultured with GDC-0980 and after four days in culture, growth inhibition was quantified by Alamar blue assay. Changes in activity of FLT-3 and various associated intracellular signaling components were evaluated by Western blot analysis. Similarly, the induction of apoptosis was measured by the activation of various caspases, Bcl-2 and PARP cleavage. Actin was used for loading control. Results: GDC-0980 induced effective cytotoxicity in all of the leukemia cell lines (mean IC50 = 0.65 uM range 0.2 - 1uM). The sensitivity was highest in cells with FLT-3 over-expression followed by ITD and then wild-type (wt). As expected, changes in the modulators of apoptosis such as caspases and PARP were noted although PARP cleavage was observable at much lower drug concentrations (0.01uM). At IC50 concentrations GDC-0980 mediated a decrease in active mTOR. The effect on markers of PI3K/mTOR activity (loss of pS6 and p-AKT) was more pronounced in cells with aberrant FLT-3 than those with wt FLT-3. Meanwhile, in all leukemic cells the levels of active PTEN, ERK, JAK and STAT-5 remained unchanged following treatment with similar drug concentrations. Unexpectedly, GDC-0980 induced FLT-3 de-phosphorylation in cells with FLT-3 over-expression and ITD, but this effect was not noted in leukemia cells with wt FLT-3. Discussion: Our data indicate the potential of the PI3K/mTOR inhibitor GDC-0980 to affect specific intracellular signaling pathways in pediatric leukemia cells with abnormal FLT-3. We also provide evidence for the unique ability of GDC-0980 to directly target constitutively active FLT-3 suggesting its effective use in specific leukemia sub-types. These data, in addition to the identification of the targets that are modified by this agent, provide rationale for additional pre-clinical experiments and the subsequent formulation of clinical studies for the treatment of such refractory leukemias in infants and children.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C175.

Citation Format: Meaghan Hagerty, Aarthi Jayanthan, Yibing Ruan, Tanya Trippett, Aru Narendran. Effective targeting of PI3K/mTOR pathways with GDC-0980 in FLT-3 mutant pediatric leukemia. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C175.

Abstract C205: Cytotoxicity and target modulation in pediatric solid tumors by the proteasome inhibitor carfilzomib

Rationale: Currently, even with escalating multimodal treatment regimens, most children with recurrent metastatic solid tumors endure unacceptably high mortality rates. Hence, there is an urgent need to identify novel targets and new therapeutic approaches. Recent studies have shown that proteasome inhibition leads to effective tumor killing in cells that have acquired treatment resistance and metastatic properties. Carfilzomib (CFZ) is a selective and potent proteasome inhibitor that binds and inhibits the 20S proteasome resulting in the accumulation of polyubiquinated proteins and consequently, cycle arrest, growth inhibition and apoptosis. Clinical trials in adult myeloma have shown activity and a high tolerability of CFZ. However, data with respect to the potential of this agent for refractory pediatric solid tumors are not yet available.

Methods: Cells from a panel of lines including neuroblastoma (n=6), Ewings sarcoma (n=2), osteosarcoma (n=2), rhabdomyosarcoma (n=4) and ATRT (n=2), were treated with increasing concentrations of CFZ and cell growth inhibition was quantified by Alamar blue assay. Drug pulsing experiments were carried out by adding new drug preparations at defined time intervals. Target modulation and apoptosis analyses were done by Western blotting. Drug combination studies were interpreted using the Chou and Talalay method.

Results and Discussion: CFZ showed effective cytotoxicity against all cell lines tested (mean IC50 = 7nM, range = 1-20nM) and activity in a fluorophore tagged cell based proteasome assay. More detailed target modulation studies in NB cells showed a dose dependent initial up-regulation of MCL-1 that subsequently decreased in a manner corresponding with PARP cleavage. Up-regulation of BCL-2 was also noted but required the exposure to higher drug concentrations. This indicated the initial stabilization or activation of survival pathways in response to drug treatment. Drug scheduling showed that the minimum exposure of 4 to 8 hours /day is needed for effective cumulative killing coinciding with MCL-1 regulation. Drug combination studies identified the ability of CFZ to synergistically enhance the activity of a number of chemotherapeutic agents, including etoposide, vincristine, mefloquine and the BCL-2 antagonist ABT-263. However, the extent of synergy differed greatly between cell types indicating the potentially variable relationship of proteasome functions to distinct oncogenic pathways present in different cells. Our studies provide initial in vitro data on the potential of CFZ to treat pediatric solid tumors and support further investigations in to the components of drug scheduling, biological correlates and drug combinations for future early phase clinical trials.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C205.

Citation Format: Yibing Ruan, David Liu, Aarthi Jayanthan, Tony Truong, Jessica Boklan, Aru Narendran. Cytotoxicity and target modulation in pediatric solid tumors by the proteasome inhibitor carfilzomib. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C205.

Double-deleted vaccinia virus in virotherapy for refractory and metastatic pediatric solid tumors

BACKGROUND

Previous studies have shown successful antitumor effects of systemically delivered double-deleted vaccinia virus (vvDD) against a number of adult tumor models, including glioma, colon and ovarian cancers. The purpose of this study was to investigate the oncolytic potential of vvDD against a panel of cell lines representative of pediatric solid tumors that are currently difficult to cure.

METHODS

Cell lines derived from central nervous system atypical teratoid rhabdoid tumor (AT/RT) (BT12, BT16 and KCCF1), sarcoma (143B, HOS, RD and RH30), and neuroblastoma (SKNAS, SKNBE2, IMR-5 and IMR-32) were examined for vvDD mediated cytotoxicity defined by virus expansion followed by loss of tumor cell viability. The normal human fibroblast cell line HS68 was used as a control. Next, relevant orthotopic, subcutaneous and lung metastasis xenograft models were treated with intravenous doses of live vvDD or killed virus controls (DV). Tumor growth inhibition and viral replication were quantified and survival outcomes of these animals were assessed.

RESULTS

vvDD was able to infect and kill nine of eleven of the pediatric tumor cells (81.8%) in vitro. In xenograft models, intravenous administration of a single dose of vvDD significantly inhibited the growth of tumors and prolonged the survival of intracranial and metastatic tumors.

CONCLUSIONS

Oncolytic vvDD administered i.v. shows activity in preclinical models of pediatric malignancies that are resistant to many currently available treatments. Our data support further evaluation of vvDD virotherapy for refractory pediatric solid tumors.

Profiling pathway-specific novel therapeutics in preclinical assessment for central nervous system atypical teratoid rhabdoid tumors (CNS ATRT): favorable activity of targeting EGFR- ErbB2 signaling with lapatinib

Despite intensifying multimodal treatments, children with central nervous system atypical teratoid/rhabdoid tumor (CNS ATRT) continue to endure unacceptably high mortality rates. At present, concerted efforts are focusing on understanding the characteristic INI1 mutation and its implications for the growth and survival of these tumors. Additionally, pharmaceutical pipeline libraries constitute a significant source of potential agents that can be taken to clinical trials in a timely manner. However, this process requires efficient target validation and relevant preclinical studies. As an initial screening approach, a panel of 129 small molecule inhibitors from multiple pharmaceutical pipeline libraries was tested against three ATRT cell lines by in vitro cytotoxicity assays. Based on these data, agents that have strong activity and corresponding susceptible cellular pathways were identified. Target modulation, antibody array analysis, drug combination and in vivo xenograft studies were performed on one of the pathway inhibitors found in this screening. Approximately 20% of agents in the library showed activity with IC(50) values of 1 μM or less and many showed IC(50) values less than 0.05 μM. Intra cell line variability was also noted among some of the drugs. However, it was determined that agents capable of affecting pathways constituting ErbB2, mTOR, proteasomes, Hsp90, Polo like kinases and Aurora kinases were universally effective against the three ATRT cell lines. The first target selected for further analysis, the inhibition of ErbB2-EGFR pathway by the small molecule inhibitor lapatinib, indicated inhibition of cell migration properties and the initiation of apoptosis. Synergy between lapatinib and IGF-IR inhibition was also demonstrated by combination index (CI) values. Xenograft studies showed effective antitumor activity of lapatinib in vivo. We present an experimental approach to identifying agents and drug combinations for future clinical trials and provide evidence for the potential of lapatinib as an effective agent in the context of the biology and heterogeneity of its targets in ATRT.

Occurrence and modulation of therapeutic targets of Aurora kinase inhibition in pediatric acute leukemia cells

Acute lymphoblastic leukemia (ALL) is one of the most prevelant pediatric malignancies. Although cure rates have improved in recent decades, approximately one in five children relapse, and survival rates post-relapse remain low. Therefore, more effective and innovative therapeutic strategies are needed in order to improve the outcome in these children. Aurora kinases, a family of serine/threonine kinases essential for regulated mitosis, are overexpressed in many forms of cancer, and have been identified as potential targets for cancer therapeutics. Based on this premise, we evaluated the activity of the Aurora-A/B inhibitor AT9283 against pediatric leukemia cells. It was found that AT9283 significantly inhibited the growth and survival of cell lines derived from patients with pediatric leukemia. Specifically, AT9283 promoted Flt-3 dephosphorylation, inhibiting the activity of downstream effectors such as Erk and Mek. In addition, apoptotic markers were also identified, providing a panel of markers for biological correlative analysis for drug activity. Lastly, drug combination studies demonstrated the potential of several novel and conventional agents to synergize with AT9283, including apicidin, 17-allylamino-17-demethoxygeldanamycin (17-AAG) and doxorubicin. These data provide a rationale for further studies and the formulation of a clinical trial of AT9283 for the treatment of refractory pediatric ALL.

Polo-like kinase 1 inhibition kills glioblastoma multiforme brain tumor cells in part through loss of SOX2 and delays tumor progression in mice

Glioblastoma multiforme (GBM) ranks among the deadliest types of cancer and given these new therapies are urgently needed. To identify molecular targets, we queried a microarray profiling 467 human GBMs and discovered that polo-like kinase 1 (PLK1) was highly expressed in these tumors and that it clustered with the proliferative subtype. Patients with PLK1-high tumors were more likely to die from their disease suggesting that current therapies are inactive against such tumors. This prompted us to examine its expression in brain tumor initiating cells (BTICs) given their association with treatment failure. BTICs isolated from patients expressed 110-470 times more PLK1 than normal human astrocytes. Moreover, BTICs rely on PLK1 for survival because the PLK1 inhibitor BI2536 inhibited their growth in tumorsphere cultures. PLK1 inhibition suppressed growth, caused G(2) /M arrest, induced apoptosis, and reduced the expression of SOX2, a marker of neural stem cells, in SF188 cells. Consistent with SOX2 inhibition, the loss of PLK1 activity caused the cells to differentiate based on elevated levels of glial fibrillary acidic protein and changes in cellular morphology. We then knocked glial fibrillary acidic protein (GFAP) down SOX2 with siRNA and showed that it too inhibited cell growth and induced cell death. Likewise, in U251 cells, PLK1 inhibition suppressed cell growth, downregulated SOX2, and induced cell death. Furthermore, BI2536 delayed tumor growth of U251 cells in an orthotopic brain tumor model, demonstrating that the drug is active against GBM. In conclusion, PLK1 level is elevated in GBM and its inhibition restricts the growth of brain cancer cells.

Multi-tyrosine kinase inhibitors in preclinical studies for pediatric CNS AT/RT: Evidence for synergy with Topoisomerase-I inhibition

BACKGROUND

Currently, Atypical Teratoid Rhabdoid Tumor (AT/RT) constitutes one of the most difficult to treat malignancies in pediatrics. Hence, new knowledge of potential targets for therapeutics and the development of novel treatment approaches are urgently needed. We have evaluated the presence of cytokine pathways and the effects of two clinically available multi-tyrosine kinase inhibitors for cytotoxicity, target modulation and drug combinability against AT/RT cell lines.

RESULTS

AT/RT cell lines expressed measurable quantities of VEGF, FGF, PDGF and SDF-1, although the absolute amounts varied between the cell lines. The targeted receptor tyrosine kinase inhibitor sorafenib inhibited the key signaling molecule Erk, which was activated following the addition of own conditioned media, suggesting the existence of autocrine/paracrine growth stimulatory pathways. The multi-tyrosine kinase inhibitors sorafenib and sunitinib also showed significant growth inhibition of AT/RT cells and their activity was enhanced by combination with the topoisomerase inhibitor, irinotecan. The loss of cytoplasmic NF-kappa-B in response to irinotecan was diminished by sorafenib, providing evidence for a possible benefit for this drug combination.

CONCLUSIONS

In addition to previously described involvement of insulin like growth factor (IGF) family of cytokines, a multitude of other growth factors may contribute to the growth and survival of AT/RT cells. However, consistent with the heterogeneous nature of this tumor, quantitative and qualitative differences may exist among different tumor samples. Multi-tyrosine kinase inhibitors appear to have effective antitumor activity against all cell lines studied. In addition, the target modulation studies and drug combinability data provide the groundwork for additional studies and support the evaluation of these agents in future treatment protocols.

Ion channels in pediatric CNS Atypical Teratoid/Rhabdoid Tumor (AT/RT) cells: potential targets for novel therapeutic agents

The central nervous system Atypical Teratoid/Rhabdoid Tumor (CNS AT/RT) is a highly malignant neoplasm that commonly affects infants and young children, and has an extremely poor prognosis. Recently, a small subset of ion channels have been found to be over-expressed in a variety of malignant cells, thus emerging as potential therapeutic targets for difficult to treat tumors. We have studied the electrophysiological properties of AT/RT cell lines with particular attention to cell volume sensitive ion channels (VSC). This class of membrane proteins can play a fundamental role in cellular processes relevant to tumor development. We have found that chloride selective VSCs are particularly active in AT/RT cell lines, compared to non-tumor cells. We evaluated specific inhibitors for activity against chloride selective VSCs and consequently for their ability to inhibit the growth and survival of AT/RT cells in vitro. The results demonstrated that the extent of volume sensitive membrane current inhibition by these agents was correlated with their potency in AT/RT cell growth inhibition in vitro. In addition, we showed that ion channel inhibition enhanced the activity of certain anti-neoplastic agents, suggesting its value in effective drug combination protocols. Results presented provide preliminary in vitro data for possible evaluation of distinct ion channels as plausible therapeutic targets in the treatment of AT/RT.

Systematic in-vitro evaluation of the NCI/NIH Developmental Therapeutics Program Approved Oncology Drug Set for the identification of a candidate drug repertoire for MLL-rearranged leukemia

Despite significant progress made in the overall cure rate, the prognosis for relapsed and refractory malignancies in children remains extremely poor. Hence, there is an urgent need for studies that enable the timely selection of appropriate agents for Phase I clinical studies. The Pediatric Oncology Experimental Therapeutics Investigators’ Consortium (POETIC) is systematically evaluating libraries of known and novel compounds for activity against subsets of high-risk pediatric malignancies with defined molecular aberrations for future clinical development. In this report, we describe the in-vitro activity of a diverse panel of approved oncology drugs against MLL-rearranged pediatric leukemia cell lines. Agents in the Approved Oncology Drug Set II (National Cancer Institute/National Institutes of Health Developmental Therapeutics Program) were evaluated by in-vitro cytotoxicity assays in pediatric acute lymphoblastic leukemia and acute myeloid leukemia cell lines with MLL gene rearrangements. Validation studies were carried out with patient leukemia cells in culture. Comparative analysis for toxicity against nonmalignant cells was evaluated in normal bone marrow stromal cells and normal human lymphocytes. Results from this study show that 42 of the 89 agents tested have measurable cytotoxicity against leukemia cells, and among these, 12 were effective against all five MLL-rearranged cell lines (IC₅₀ [half maximal inhibitory concentration] < 1 μM). These 12 agents include cladribine, dactinomycin, daunorubicin, docetaxel, etoposide, gemcitabine, mitomycin C, mitoxantrone, teniposide, topotecan, triethylenemelamine, and vinblastine. We show that the Approved Oncology Drug Set II contains a number of agents with potent antileukemic activity in the tested cell lines. As approved drugs, these agents have been used in clinical settings for many years for other malignancies, thus their toxicity profile, pharmacokinetics, and other properties are readily available. Further evaluation of their use in future clinical trials for pediatric leukemia with MLL abnormalities should be considered.

Cytotoxicity, drug combinability, and biological correlates of ABT-737 against acute lymphoblastic leukemia cells with MLL rearrangement

BACKGROUND

ABT-737 is a BH3 mimetic small-molecule inhibitor that binds with high affinity to Bcl-2 to induce apoptosis in malignant cells and has shown promise as an effective anti-leukemic agent in pediatric preclinical tests. This study focuses on the effects of ABT-737 on leukemia cells with MLL rearrangement and identifies some of the biological correlates of its activity.

PROCEDURE

Cells were cultured in the presence of increasing concentrations of ABT-737 alone or in combination with other agents. After 4 days in culture, cell growth inhibition was measured by Alamar blue assay. The expression and activation of potential intracellular targets of ABT-737 activity were determined by Western blot analysis.

RESULTS

Significant Bcl-2 expression was detected in all infant leukemia cells investigated. ABT-737 induced cell death in all cell lines studied although the IC(50) values differed somewhat between cell lines. Western blot analysis identified the effects of ABT-737 on survival and apoptosis-regulatory proteins PARP, caspase-8, and cytochrome-c. Drug combination studies indicated synergy with distinct anti-neoplastic agents, including the multi-tyrosine kinase inhibitor sunitinib. This effective drug synergy appears to be mediated by the combined inhibition of Bcl-2 and intracellular signaling pathways.

CONCLUSIONS

We describe the in vitro studies to demonstrate the activity and drug combinability of ABT-737 against MLL rearranged leukemia cells. In addition, identification of the molecular changes that occur in the presence of ABT-737 provides information regarding effective target validation and target modulation analyses in future clinical trials.

Analysis of multiple growth regulatory proteins using dissociable staining antibody arrays on solid tumor biopsy specimens

Growth of tumor cells is often a function of deregulated growth factor receptors and their corresponding intracellular signalling molecules. The dissociable antibody staining arrays have the versatility to rapidly identify the expression, activation, and localization of such molecules and pathways in biopsy specimens. This report describes a protocol to quantify the activity of a panel of signalling molecules in Wilms tumor biopsy specimens and surrounding nonmalignant renal cells. We propose that this technique can be used to rapidly identify multiple markers and may aid in the study of aberrant growth regulatory mechanisms and potential targets for therapeutics from pathologic specimens.

Childhood acute lymphoblastic leukemia (ALL) presenting with severe osteolysis: a model to study leukemia-bone interactions and potential targeted therapeutics

Interference with the molecular mechanisms that generate tumor supportive niches in the bone microenvironment is a rational approach to inhibit the growth of hematological malignancies. However, the advancement of knowledge in this area has been slowed down by the lack of in vitro models to facilitate the screening of potential candidate agents. The rare cases of acute lymphoblastic leukemia (ALL) in children presenting with extensive bone involvement may represent an exaggerated form of some aspects of the normal tumor-bone interactions. Thus, these cases can provide insight into processes that are otherwise challenging to uncover. The authors describe the case of a 6-year-old child who presented with severe osteopenia that resolved at the time of leukemic remission. Compared to control sera, serum taken at disease presentation contained increased levels of a group of osteolytic cytokines and was effective in activating preosteoclast cells in culture. Based on these findings, the authors describe an experimental model to identify agents that would interfere with leukemia mediated osteolytic process.

Establishment of atypical-teratoid/rhabdoid tumor (AT/RT) cell cultures from disseminated CSF cells: a model to elucidate biology and potential targeted therapeutics

Atypical teratoid/rhabdoid tumor (AT/RT) is a highly malignant central nervous system neoplasm that usually affects infants and young children. In this report, we describe culture conditions that enabled the sustained growth of tumor cells obtained from the cerebrospinal fluid (CSF) of an infant with AT/RT. These cells retained the morphological and biomarker characteristics of the original tumor. A screening of receptor tyrosine kinases identified the presence of phosphorylated ErbB4, Insulin-R, PDGFR and IGF-IR, which appear to depend on Hsp90 to maintain their active form. IGF-IR activity is consistent with data from other established AT/RT cell lines. Inhibition of IGF-IR by the small molecular weight inhibitor AEW541 led to growth suppression of cultured AT/RT cells. In addition, neutralizing antibodies to IGF-II also inhibited the growth of these cells suggesting a potential autocrine function for this cytokine. We also compared cultured AT/RT cells to established cell lines to identify consistent drug sensitivity patterns among these cells. In addition to previously described cell lines and xenograft models, continuous culture of CSF derived cells may also provide an effective way to study the biology of AT/RT and to identify potential targets for future therapeutics for this tumor.

Effects of Hsp90 inhibition in neuroblastoma: analysis of drug sensitivity, target modulation and the influence of bone marrow microenvironment

Heat shock protein 90 (Hsp90) safeguards the structural integrity and function of many of the key growth regulatory proteins found in malignant cells. Consequently, among the new generation targeted therapeutics, heat shock protein inhibitors have the unique property of being able to target an expansive array of divergent molecular mechanisms involved in cancer growth and metastasis. 17-N-Allylamino-17-demethoxygeldanamycin (17-AAG) is one such agent that has been shown to bind to Hsp90 and thus reduce the stability and activity of many key growth regulatory molecules and pathways. A number of recent clinical trials have investigated the maximum tolerated dose, toxicity and pharmacokinetic profiles of 17-AAG in pediatric patients with recurrent tumors. In this study, we describe the effects of 17-AAG against a panel of neuroblastoma (NB) cell lines with respect to cytotoxicity, target modulation and inhibition of vascular endothelial growth factor (VEGF) expression. 17-AAG was found to inhibit the growth of all NB cell lines tested, though effective inhibitory concentrations varied among cell lines. 17-AAG also suppressed the expression of VEGF. The cytotoxic effect of 17-AAG on tumor cells was diminished when co-cultured with bone marrow stromal cells suggesting a potential role for the microenvironment in tumor drug interactions. Findings from target modulation analysis as well as drug combination assays provide a frame-work to formulate effective protocols for the treatment of NB.