Abstract 923: Analysis of microtubule perturbations and androgen receptor localization in circulating tumor cells from castration resistant prostate cancer patients as predictive biomarkers of clinical response to docetaxel chemotherapy

Although the recent availability of novel treatment for castration resistant prostate cancer (CRPC) has shown improvements in overall survival, CRPC is incurable and lethal disease. It has been recognized that tumor progression despite castrate levels of androgens is still associated with active signaling from the androgen receptor (AR) which affects gene transcription following its nuclear accumulation. Taxanes bind to and stabilize cellular microtubules (MTs) perturbing the fine and intricate organization of the microtubule network thus, impairing MT-based cell processes like cell division and intracellular trafficking. Our recent work showed that AR translocation from the cytoplasm to the nucleus is MT-dependent and that taxane treatment sequesters AR into the cytoplasm inhibiting the receptors transcriptional activity. While this novel mechanism of taxane activity may explain the clinical efficacy of taxanes in CRPC, clinically we still fail to understand the molecular basis of patient response or resistance to taxanes. We hypothesized that engagement of the MT-AR pathway by the taxanes can be used as read-out of effective drug-target engagement in cancer cells and can be used as biomarker predictive of taxane activity in CRPC patients. To assess the predictive role of these biomarkers, we are currently conducting a prospective clinical study in which we isolate circulating tumor cells (CTCs) from the peripheral blood of CRPC patient undergoing docetaxel treatment. We have enrolled 50 of the 80 total patients, from which we isolated CTCs before (at baseline) and during docetaxel treatment (at day 8 of first cycle of docetaxel and upon relapse) using two different approaches: the EpCAM-based immunomagnetic enrichment (CellSearch) and the ficolling technique to isolate unenriched PBMCs inclusive of CTCs. To avoid issues with leucocyte contamination we subject the EpCAM captured cells are to staining for DAPI , PSMA, CD-45, AR and Tubulin and image them by confocal microscopy. CTCs are defined as nucleated, PSMA+, CD-45- cells. The unenriched PBMC population is also subjected to the same multiplex confocal microscopy protocol. Microtubule network morphology and AR subcellular localization in then assessed in CTCs and each biomarker alone and in combination is correlated with clinical response to docetaxel treatment defined by PCWG2 recommendation. So far we have collected and analyzed baseline and cycle 1-Day 8 samples from all 50 patients and relapse samples from 31 patients. Determination of effective drug-target engagement on C1D8 may allow early detection of molecular response to treatment, or lack of molecular response, which will ultimately allow for chemotherapy customization for the individual patient. Citation Format: Shinsuke Tasaki, Matthew Sung, Alexandre Matov, Giuseppe Galletti, Elan Diamond, Neil Bander, Kathy Zhou, Scott Tagawa, David Nanus, Paraskevi Giannakakou. Analysis of microtubule perturbations and androgen receptor localization in circulating tumor cells from castration resistant prostate cancer patients as predictive biomarkers of clinical response to docetaxel chemotherapy. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 923. doi:10.1158/1538-7445.AM2014-923