Austin J. Combest

Multiple Roles of Cyclin-Dependent Kinase 4/6 Inhibitors in Cancer Therapy

BACKGROUND Cyclin-dependent kinases (CDKs) regulate cell proliferation and coordinate the cell cycle checkpoint response to DNA damage. Although inhibitors with varying selectivity to specific CDK family members have been developed, selective CDK4/6 inhibitors have emerged as the most attractive antineoplastic agents because of the importance of CDK4/6 activity in regulating cell proliferation and the toxic effects associated with inhibition of other CDKs (eg, CDK1 and CDK2). METHODS FVB/N wild-type mice (n = 13) were used to evaluate carboplatin-induced myelosuppression in bone marrow by complete blood cell counts after treatment with the CDK4/6 inhibitor PD0332991. Genetically engineered murine models of retinoblastoma (Rb)-competent (MMTV-c-neu) and Rb-incompetent (C3-TAg) breast cancer (n = 16 MMTV-c-neu mice in the carboplatin plus vehicle control group, n = 17 MMTV-c-neu mice in the carboplatin plus PD0332991 group, n = 17 C3-TAg mice in the carboplatin plus vehicle control group, and n = 14 C3-TAg mice in the carboplatin plus PD0332991 group) were used to investigate the antitumor activity of PD0332991 alone or in combination with chemotherapy. All statistical tests were two-sided. RESULTS Coadministration of PD0332991 with carboplatin compared with carboplatin alone in FVB/N wild-type mice increased hematocrit (51.2% vs 33.5%, difference = 17.7%, 95% confidence interval [CI] = -26.7% to -8.6%, P < .001), platelet counts (1321 vs 758.5 thousand cells per μL, difference = 562.5 thousand cells per μL, 95% CI = -902.8 to -222.6, P = .002), myeloid cells (granulocytes and monocytes; 3.1 vs 1.6 thousand cells per μL, difference = 1.5 thousand cells per μL, 95% CI = -2.23 to -0.67, P < .001), and lymphocytes (7.9 vs 5.4 thousand cells per μL, difference = 2.5 thousand cells per μL, 95% CI = -4.75 to -0.18, P = .02). Daily administration of PD0332991 exhibited antitumor activity in MMTV-c-neu mice as a single agent. However, the combination of carboplatin plus PD0332991 decreased antitumor activity compared with carboplatin alone in Rb-competent mice (mean percent change in tumor volume at day 21 = -52.6% vs 3.7% for carboplatin and carboplatin plus PD0332991, respectively, difference = 56.3%, 95% CI = -109.0% to -3.6%, P = .04). In contrast, Rb-deficient tumors in C3-Tag mice were resistant to PD0332991, and coadministration of PD0332991 plus carboplatin had no effect on in vivo tumor growth (mean percent change in tumor volume at day 21 = 118.8% and 109.1% for carboplatin and carboplatin plus PD0332991, respectively, difference = 9.7%, 95% CI = -183.5% to 202.9%, P = .92). Finally, in tumor-bearing mice, coadministration of PD0332991 with carboplatin provided statistically significant protection of platelets (P = .04). CONCLUSION We believe that the present data support a possible role for CDK4/6 inhibitors in a majority of patients with advanced cancer: to either inhibit tumor growth in CDK4/6-dependent tumors or ameliorate the dose-limiting toxicities of chemotherapy in CDK4/6-indepdendent tumors. Our data also suggest CDK4/6 inhibitors should not be combined with DNA-damaging therapies, such as carboplatin, to treat tumors that require CDK4/6 activity for proliferation.

Combined PI3K/mTOR and MEK Inhibition Provides Broad Antitumor Activity in Faithful Murine Cancer Models

Purpose: Anticancer drug development is inefficient, but genetically engineered murine models (GEMM) and orthotopic, syngeneic transplants (OST) of cancer may offer advantages to in vitro and xenograft systems. Experimental Design: We assessed the activity of 16 treatment regimens in a RAS-driven, Ink4a/Arf-deficient melanoma GEMM. In addition, we tested a subset of treatment regimens in three breast cancer models representing distinct breast cancer subtypes: claudin-low (T11 OST), basal-like (C3-TAg GEMM), and luminal B (MMTV-Neu GEMM). Results: Like human RAS-mutant melanoma, the melanoma GEMM was refractory to chemotherapy and single-agent small molecule therapies. Combined treatment with AZD6244 [mitogen-activated protein–extracellular signal-regulated kinase kinase (MEK) inhibitor] and BEZ235 [dual phosphoinositide-3 kinase (PI3K)/mammalian target of rapamycin (mTOR) inhibitor] was the only treatment regimen to exhibit significant antitumor activity, showed by marked tumor regression and improved survival. Given the surprising activity of the “AZD/BEZ” combination in the melanoma GEMM, we next tested this regimen in the “claudin-low” breast cancer model that shares gene expression features with melanoma. The AZD/BEZ regimen also exhibited significant activity in this model, leading us to testing in even more diverse GEMMs of basal-like and luminal breast cancer. The AZD/BEZ combination was highly active in these distinct breast cancer models, showing equal or greater efficacy compared with any other regimen tested in studies of over 700 tumor-bearing mice. This regimen even exhibited activity in lapatinib-resistant HER2+ tumors. Conclusion: These results show the use of credentialed murine models for large-scale efficacy testing of diverse anticancer regimens and predict that combinations of PI3K/mTOR and MEK inhibitors will show antitumor activity in a wide range of human malignancies. Clin Cancer Res; 18(19); 5290–303. ©2012 AACR.

Predicting Drug Responsiveness in Human Cancers Using Genetically Engineered Mice

Purpose: To use genetically engineered mouse models (GEMM) and orthotopic syngeneic murine transplants (OST) to develop gene expression-based predictors of response to anticancer drugs in human tumors. These mouse models offer advantages including precise genetics and an intact microenvironment/immune system. Experimental Design: We examined the efficacy of 4 chemotherapeutic or targeted anticancer drugs, alone and in combination, using mouse models representing 3 distinct breast cancer subtypes: Basal-like (C3(1)-T-antigen GEMM), Luminal B (MMTV-Neu GEMM), and Claudin-low (T11/TP53−/− OST). We expression-profiled tumors to develop signatures that corresponded to treatment and response, and then tested their predictive potential using human patient data. Results: Although a single agent exhibited exceptional efficacy (i.e., lapatinib in the Neu-driven model), generally single-agent activity was modest, whereas some combination therapies were more active and life prolonging. Through analysis of RNA expression in this large set of chemotherapy-treated murine tumors, we identified a pair of gene expression signatures that predicted pathologic complete response to neoadjuvant anthracycline/taxane therapy in human patients with breast cancer. Conclusions: These results show that murine-derived gene signatures can predict response even after accounting for common clinical variables and other predictive genomic signatures, suggesting that mice can be used to identify new biomarkers for human patients with cancer. Clin Cancer Res; 19(17); 4889–99. ©2013 AACR.

Genetically Engineered Cancer Models, But Not Xenografts, Faithfully Predict Anticancer Drug Exposure in Melanoma Tumors

BACKGROUND Rodent studies are a vital step in the development of novel anticancer therapeutics and are used in pharmacokinetic (PK), toxicology, and efficacy studies. Traditionally, anticancer drug development has relied on xenograft implantation of human cancer cell lines in immunocompromised mice for efficacy screening of a candidate compound. The usefulness of xenograft models for efficacy testing, however, has been questioned, whereas genetically engineered mouse models (GEMMs) and orthotopic syngeneic transplants (OSTs) may offer some advantages for efficacy assessment. A critical factor influencing the predictability of rodent tumor models is drug PKs, but a comprehensive comparison of plasma and tumor PK parameters among xenograft models, OSTs, GEMMs, and human patients has not been performed. METHODS In this work, we evaluated the plasma and tumor dispositions of an antimelanoma agent, carboplatin, in patients with cutaneous melanoma compared with four different murine melanoma models (one GEMM, one human cell line xenograft, and two OSTs). RESULTS Using microdialysis to sample carboplatin tumor disposition, we found that OSTs and xenografts were poor predictors of drug exposure in human tumors, whereas the GEMM model exhibited PK parameters similar to those seen in human tumors. CONCLUSIONS The tumor PKs of carboplatin in a GEMM of melanoma more closely resembles the tumor disposition in patients with melanoma than transplanted tumor models. GEMMs show promise in becoming an improved prediction model for intratumoral PKs and response in patients with solid tumors.

Challenges in the development of an autologous heat shock protein based anti-tumor vaccine.

In spite of its long history, immunotherapy of cancer has led to only a few regulatory approved treatments, starting with interleukin (IL)-2 in renal cancer and melanoma and interferon in melanoma.1-4 Sipuleucel-T was approved for castrate-resistant prostate cancer in 2010 and ipilimumab and pegylated interferon-α2b in 2011 for the treatment of metastatic and adjuvant melanoma respectively.5-7 Evidence of both humoral and cellular immune recognition of human cancer has been found and supports the hypothesis that specific autologous anti-tumor activity exists and may be enhanced with therapeutic activity.8 Published data from trials with autologous tumor vaccines documented activity although only one autologous-like vaccine has been approved in the US.5,9 The concept of enhancing specific innate anti-tumor activity is attractive and the approach developed by Srivastava et al., which is designed to present a unique peptide profile of each autologous tumor to the host immune system, using a heat shock protein fraction as both carrier and adjuvant, is conceptually appealing. In syngeneic rat tumors, this approach was shown to lead to anti-tumor activity and as described below evidence of clinical activity was found in clinical trials. Vitespen (also known as Oncophage or HSPPC-96), is an immunotherapeutic agent derived from the tissue of a patient’s own tumor. Vitespen is a heat-shock protein (HSP) (glycoprotein 96)-peptide complex that is purified ex vivo from an individual patient’s tumor cells through preparative chromatography.10 Vitespen failed to show broad activity in randomized clinical trials despite encouraging results in select patients. In this commentary, we highlight the clinical trial experience with vitespen, comment on potential reasons for the limited success and offer suggestions for future tumor vaccine development strategies.

Phase Ib/II trial of NC-6004 (nanoparticle cisplatin) plus gemcitabine in patients with advanced solid tumors

Purpose: NC-6004, a novel cisplatin nanoparticle developed using micellar technology exhibits sustained release of cisplatin and selective distribution to tumors. Preclinical data demonstrated a favorable tolerability profile and preserved or improved antitumor activity compared with cisplatin across animal models. We evaluated the safety and tolerability of NC-6004 and gemcitabine using a Bayesian continual reassessment model (N-CRM) to determine the optimal dose. Experimental Design: Patients with advanced solid tumors received NC-6004 at 60 to 180 mg/m2 on day 1 and gemcitabine at 1,250 mg/m2 on days 1 and 8 every 3 weeks. Dose escalation of NC-6004 began with a single patient run-in until a dose-limiting toxicity occurred at 180 mg/m2. Cohorts of four patients were enrolled at doses predicted by the N-CRM. The maximum tolerated dose (MTD) was defined as having the greatest probability of target toxicity <25%. Quality of life was assessed using EORTC-QLQ-C30. Results: Among 22 patients, the most common grade III/IV hematologic adverse events were leukopenia (68%) and thrombocytopenia (59%). Of 20 pretreated patients evaluable for response, half were previously exposed to a platinum agent. The MTD was 135 mg/m2. Nine patients were treated at the MTD with median treatment duration of 15 weeks (range, 3–50). Tumor shrinkage occurred in 11 (55%), partial responses in 3 (15%), and stable disease in 14 (70%). Most patients reported stable or improved EORTC QLQ-C30 scores. Conclusions: Greater cisplatin equivalent doses were achieved with no clinically significant neuro-, oto-, or nephrotoxicity. These data demonstrate tolerability and promising activity of NC-6004 in combination with gemcitabine. Clin Cancer Res; 24(1); 43–51. ©2017 AACR.