Jessica Teh

An in vivo reporter to quantitatively and temporally analyze the effects of CDK4/6 inhibitor-based therapies in melanoma

Aberrant cell-cycle progression is a hallmark feature of cancer cells. Cyclin-dependent kinases 4 and 6 (CDK4/6) drive progression through the G1 stage of the cell cycle, at least in part, by inactivating the tumor suppressor, retinoblastoma. CDK4/6 are targetable and the selective CDK4/6 inhibitor, palbociclib, was recently FDA approved for the treatment of estrogen receptor-positive, HER2-negative advanced breast cancer. In cutaneous melanoma, driver mutations in NRAS and BRAF promote CDK4/6 activation, suggesting that inhibitors such as palbociclib are likely to provide therapeutic benefit in combination with BRAF inhibitors and/or MEK inhibitors that are FDA-approved. However, the determinants of the response to CDK4/6 inhibitors alone and in combination with other targeted inhibitors are poorly defined. Furthermore, in vivo systems to quantitatively and temporally measure the efficacy of CDK4/6 inhibitors and determine the extent that CDK activity is reactivated during acquired resistance are lacking. Here, we describe the heterogeneous effects of CDK4/6 inhibitors, the expression of antiapoptotic proteins that associate with response to CDK4/6 and MEK inhibitors, and the development of a luciferase-based reporter system to determine the effects of CDK4/6 inhibitors alone and in combination with MEK inhibitors in melanoma xenografts. These findings are likely to inform on-going and future clinical trials utilizing CDK4/6 inhibitors in cutaneous melanoma. Cancer Res; 76(18); 5455-66. ©2016 AACR.

A preexisting rare PIK3CAe545k subpopulation confers clinical resistance to MEK plus CDK4/6 inhibition in NRAS melanoma and is dependent on S6K1 signaling

Combined MEK and CDK4/6 inhibition (MEKi + CDK4i) has shown promising clinical outcomes in patients with NRAS-mutant melanoma. Here, we interrogated longitudinal biopsies from a patient who initially responded to MEKi + CDK4i therapy but subsequently developed resistance. Whole-exome sequencing and functional validation identified an acquired PIK3CAE545K mutation as conferring drug resistance. We demonstrate that PIK3CAE545K preexisted in a rare subpopulation that was missed by both clinical and research testing, but was revealed upon multiregion sampling due to PIK3CAE545K being nonuniformly distributed. This resistant population rapidly expanded after the initiation of MEKi + CDK4i therapy and persisted in all successive samples even after immune checkpoint therapy and distant metastasis. Functional studies identified activated S6K1 as both a key marker and specific therapeutic vulnerability downstream of PIK3CAE545K-induced resistance. These results demonstrate that difficult-to-detect preexisting resistance mutations may exist more often than previously appreciated and also posit S6K1 as a common downstream therapeutic nexus for the MAPK, CDK4/6, and PI3K pathways.Significance: We report the first characterization of clinical acquired resistance to MEKi + CDK4i, identifying a rare preexisting PIK3CAE545K subpopulation that expands upon therapy and exhibits drug resistance. We suggest that single-region pretreatment biopsy is insufficient to detect rare, spatially segregated drug-resistant subclones. Inhibition of S6K1 is able to resensitize PIK3CAE545K-expressing NRAS-mutant melanoma cells to MEKi + CDK4i. Cancer Discov; 8(5); 556-67. ©2018 AACR.See related commentary by Sullivan, p. 532See related article by Teh et al., p. 568This article is highlighted in the In This Issue feature, p. 517.

In Vivo E2F Reporting Reveals Efficacious Schedules of MEK1/2–CDK4/6 Targeting and mTOR–S6 Resistance Mechanisms

Targeting cyclin-dependent kinases 4/6 (CDK4/6) represents a therapeutic option in combination with BRAF inhibitor and/or MEK inhibitor (MEKi) in melanoma; however, continuous dosing elicits toxicities in patients. Using quantitative and temporal in vivo reporting, we show that continuous MEKi with intermittent CDK4/6 inhibitor (CDK4/6i) led to more complete tumor responses versus other combination schedules. Nevertheless, some tumors acquired resistance that was associated with enhanced phosphorylation of ribosomal S6 protein. These data were supported by phospho-S6 staining of melanoma biopsies from patients treated with CDK4/6i plus targeted inhibitors. Enhanced phospho-S6 in resistant tumors provided a therapeutic window for the mTORC1/2 inhibitor AZD2014. Mechanistically, upregulation or mutation of NRAS was associated with resistance in in vivo models and patient samples, respectively, and mutant NRAS was sufficient to enhance resistance. This study utilizes an in vivo reporter model to optimize schedules and supports targeting mTORC1/2 to overcome MEKi plus CDK4/6i resistance.Significance: Mutant BRAF and NRAS melanomas acquire resistance to combined MEK and CDK4/6 inhibition via upregulation of mTOR pathway signaling. This resistance mechanism provides the preclinical basis to utilize mTORC1/2 inhibitors to improve MEKi plus CDK4/6i drug regimens. Cancer Discov; 8(5); 568-81. ©2018 AACR.See related commentary by Sullivan, p. 532See related article by Romano et al., p. 556This article is highlighted in the In This Issue feature, p. 517.

Playing the melanoma endgame

Treatments for melanoma are of two main types: targeted therapies and immune checkpoint inhibitors. However, both are effective in only a subset of patients and are limited by acquired resistance. Here, the authors present the preclinical basis to broadly target different forms of therapy-resistant melanoma. Clin Cancer Res; 24(19); 4629–30. ©2018 AACR. See related article by Zhang et al., p. 4771

Abstract 5199: An in vivo reporter to quantitatively and temporally analyze the effects of CDK4/6 inhibitor-based therapies in melanoma

Aberrant cell cycle progression is a hallmark feature of cancer cells. Cyclin-dependent kinase 4 and 6 (CDK4/6) drive progression through the G1 stage of the cell cycle, at least in part, by inactivating the tumor suppressor, retinoblastoma (RB). CDK4/6 are targetable and the selective CDK4/6 inhibitor, palbociclib (IBRANCE/PD-0332991), was recently FDA approved for the treatment of estrogen receptor-positive, HER2-negative advanced breast cancer. In cutaneous melanoma, driver mutations in NRAS and BRAF signal to promote CDK4/6 activation suggesting that inhibitors such as palbociclib are likely to provide therapeutic benefit most likely in combination with BRAF and/or MEK inhibitors that are FDA-approved. However, the determinants of the response to CDK4/6 inhibitors alone and in combination with other targeted inhibitors are poorly defined. Furthermore, in vivo systems to quantitatively and temporally measure the efficacy of CDK4/6 inhibitors and determine the extent that CDK activity is reactivated during acquired resistance are lacking. Here, we describe the heterogeneous effects of CDK4/6 inhibitors, the expression of anti-apoptotic proteins that associate with response to CDK4/6 and MEK inhibitors, and the development of a luciferase-based reporter system to determine the effects of CDK4/6 inhibitors alone and in combination with MEK inhibitors in melanoma xenografts. These findings are likely to inform on-going and future clinical trials utilizing CDK4/6 inhibitors in cutaneous melanoma. Citation Format: Jessica L. Teh, Timothy Purwin, Evan J. Greenawalt, Inna Chervoneva, Allison Goldberg, Michael E. Davies, Andrew Aplin. An in vivo reporter to quantitatively and temporally analyze the effects of CDK4/6 inhibitor-based therapies in melanoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 5199.

Abstract 3096: CDK4/6 as a therapeutic target in malignant melanoma

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Melanoma is the most deadly form of skin cancer and FDA-approved therapies such as vemurafenib and ipilimumab are associated with short-term responses and/or severe toxicities for patients. Recent favorable clinical data with cyclin-dependent kinase (CDK) 4/6 inhibitor, palbociclib (PD’991) in ER+ breast cancer has rekindled an interest in targeting cell cycle progression to stop aberrant cell growth. Thus, we set out to analyze the potential use of CDK4/6 inhibitors in melanoma. Due to the inability of CDK4/6 inhibitors to induce apoptosis as a single agent, we explored viable combinatorial strategies that will trigger both pro-senescent and pro-apoptotic pathways that may lead to an effective treatment modality for melanoma patients. Here, we demonstrate the benefit of combining a MEK inhibitor, trametinib (GSK’212) with PD’991 both in vitro and in vivo. We did not observe any enhanced effects of combining both GSK’212 and PD’991 in cells that are wild-type for BRAF and NRAS or an RB1-null BRAF mutant line. However, B-RAF and N-RAS mutant cells exhibited an enhanced response to the combinatorial agents as compared to single agents. Furthermore, we observed a dramatic increase in apoptosis in cell lines that are sensitive to combined GSK’212 and PD’991 treatment. Expression of Inhibitor-of-Apoptosis (IAP) protein, survivin, was associated with response. Finally, because it is important to understand how resistance develops, we have validated a novel E2F reporter system in vivo to quantitatively and temporally measure the efficacy of CDK4/6 inhibitors and possible re-activation of the pathway during acquired drug resistance. Citation Format: Jessica L. Teh, Andrew Aplin. CDK4/6 as a therapeutic target in malignant melanoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3096. doi:10.1158/1538-7445.AM2015-3096