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Purpose: To assess the preclinical efficacy, clinical safety and efficacy, and MTD of palbociclib plus nab-paclitaxel in patients with advanced pancreatic ductal adenocarcinoma (PDAC). Experimental Design: Preclinical activity was tested in patient-derived xenograft (PDX) models of PDAC. In the open-label, phase I clinical study, the dose-escalation cohort received oral palbociclib initially at 75 mg/day (range, 50‒125 mg/day; modified 3+3 design; 3/1 schedule); intravenous nab-paclitaxel was administered weekly for 3 weeks/28-day cycle at 100‒125 mg/m2. The modified dose–regimen cohorts received palbociclib 75 mg/day (3/1 schedule or continuously) plus nab-paclitaxel (biweekly 125 or 100 mg/m2, respectively). The prespecified efficacy threshold was 12-month survival probability of ≥65% at the MTD. Results: Palbociclib plus nab-paclitaxel was more effective than gemcitabine plus nab-paclitaxel in three of four PDX models tested; the combination was not inferior to paclitaxel plus gemcitabine. In the clinical trial, 76 patients (80% received prior treatment for advanced disease) were enrolled. Four dose-limiting toxicities were observed [mucositis (n = 1), neutropenia (n = 2), febrile neutropenia (n = 1)]. The MTD was palbociclib 100 mg for 21 of every 28 days and nab-paclitaxel 125 mg/m2 weekly for 3 weeks in a 28-day cycle. Among all patients, the most common all-causality any-grade adverse events were neutropenia (76.3%), asthenia/fatigue (52.6%), nausea (42.1%), and anemia (40.8%). At the MTD (n = 27), the 12-month survival probability was 50% (95% confidence interval, 29.9–67.2). Conclusions: This study showed the tolerability and antitumor activity of palbociclib plus nab-paclitaxel treatment in patients with PDAC; however, the prespecified efficacy threshold was not met. Trial Registration: Pfizer Inc (NCT02501902) Significance: In this article, the combination of palbociclib, a CDK4/6 inhibitor, and nab-paclitaxel in advanced pancreatic cancer evaluates an important drug combination using translational science. In addition, the work presented combines preclinical and clinical data along with pharmacokinetic and pharmacodynamic assessments to find alternative treatments for this patient population.

Purpose:To assess the preclinical efficacy, clinical safety and efficacy, and MTD of palbociclib plus nab-paclitaxel in patients with advanced pancreatic ductal adenocarcinoma (PDAC).Experimental Design:Preclinical activity was tested in patient-derived xenograft (PDX) models of PDAC. In the open-label, phase I clinical study, the dose-escalation cohort received oral palbociclib initially at 75 mg/day (range, 50‒125 mg/day; modified 3+3 design; 3/1 schedule); intravenous nab-paclitaxel was administered weekly for 3 weeks/28-day cycle at 100‒125 mg/m2. The modified dose–regimen cohorts received palbociclib 75 mg/day (3/1 schedule or continuously) plus nab-paclitaxel (biweekly 125 or 100 mg/m2, respectively). The prespecified efficacy threshold was 12-month survival probability of ≥65% at the MTD.Results:Palbociclib plus nab-paclitaxel was more effective than gemcitabine plus nab-paclitaxel in three of four PDX models tested; the combination was not inferior to paclitaxel plus gemcitabine. In the clinical trial, 76 patients (80% received prior treatment for advanced disease) were enrolled. Four dose-limiting toxicities were observed [mucositis (n = 1), neutropenia (n = 2), febrile neutropenia (n = 1)]. The MTD was palbociclib 100 mg for 21 of every 28 days and nab-paclitaxel 125 mg/m2 weekly for 3 weeks in a 28-day cycle. Among all patients, the most common all-causality any-grade adverse events were neutropenia (76.3%), asthenia/fatigue (52.6%), nausea (42.1%), and anemia (40.8%). At the MTD (n = 27), the 12-month survival probability was 50% (95% confidence interval, 29.9–67.2).Conclusions:This study showed the tolerability and antitumor activity of palbociclib plus nab-paclitaxel treatment in patients with PDAC; however, the prespecified efficacy threshold was not met.Trial Registration:Pfizer Inc (NCT02501902)Significance:In this article, the combination of palbociclib, a CDK4/6 inhibitor, and nab-paclitaxel in advanced pancreatic cancer evaluates an important drug combination using translational science. In addition, the work presented combines preclinical and clinical data along with pharmacokinetic and pharmacodynamic assessments to find alternative treatments for this patient population.

Supplementary Materials and Methods

Supplementary Table S3 from A Preclinical and Phase Ib Study of Palbociclib plus Nab-Paclitaxel in Patients with Metastatic Adenocarcinoma of the Pancreas

Supplementary Table S1 from A Preclinical and Phase Ib Study of Palbociclib plus Nab-Paclitaxel in Patients with Metastatic Adenocarcinoma of the Pancreas

Treatment-Related Any-Grade AEs (≥10% of Patients Overall).

Summary of PFS and OS by Mutation Status.

Palbociclib Pharmacokinetic Parameters in the MTD Cohort.

Palbociclib (PAL) is approved for the treatment of HR+, HER2- ABC in combination with an aromatase inhibitor as initial endocrine based therapy in postmenopausal women or with fulvestrant in women with disease progression following endocrine therapy based on major progression-free survival (PFS) improvement in phase III studies. An exposure response analysis was conducted to evaluate how the changes in PAL exposure (PAL-E), e.g. due to the dose modifications, would affect the PFS. Data from PALOMA 2, a phase 3 study comparing the efficacy and safety of letrozole plus PAL (PAL+L) to letrozole plus placebo (L) in ABC patients (PTs), were used. The apparent PAL clearance (CL/F) for each PT was obtained from a population pharmacokinetic analysis. The individual PAL-E over the entire treatment (Cavg) was calculated using average daily dose intensity (ADI) and CL/F. PTs in the PAL+L group were divided into 4 quantiles (Q1, Q2, Q3, and Q4) based on their Cavg, and the median PFS in each quantile was compared to that of L using Kaplan-Meier plot. To account for the changes in PAL-E over time due to dose modifications during treatment, a time-varying PAL-E (Cavgt) was calculated using ADI and CL/F at each PFS time. The relationship between PFS and Cavgt was evaluated using Cox proportional hazard model. Univariate analyses were first conducted to identify any significant prognostic factors and the effect of Cavgt on PFS. Multivariate analysis was conducted to evaluate relationship between PFS and Cavgt when all significant prognostic factors identified from the univariate analysis were included in the model to account for their potential confounding effects. The median PFS were 14.5, 24.9, 27.7, 25.7, and 24.0 months for patients in the L, Q1, Q2, Q3, and Q4, respectively, indicating similar PFS across all exposure quartiles. The identified significant prognostic factors from univariate analysis were Ki67 score, age, baseline AST, baseline tumor size, baseline alkaline phosphatase, and baseline albumin levels. The estimated coefficients for Cavgt were -0.00377 and -0.00224 in univariate and multivariate analysis, respectively. The effect of PAL-E on PFS was not statistically significant (p PFS was not found to be associated with Cavgt, suggesting that PTs with different PAL-E in P+L benefited similarly and the changes in PAL-E resulted from dose modifications in the trial would not significantly affect the efficacy. However, the impacts on efficacy are unknown for any other unstudied dose modifications algorithm. Citation Format: Zheng J, Yu Y, Durairaj C, Amantea M, Dieras V, Finn R, Wang D. Palbociclib exposure-response analyses in the treatment of hormone-receptor positive (HR+), human epidermal growth factor receptor 2 negative (HER2–) advanced breast cancer (ABC) [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-21-21.

Background: Metastatic triple-negative breast cancer (TNBC) is an aggressive and heterogeneous cancer with limited therapeutic options. PARP inhibitors (PARPi) are approved to treat breast cancer harboring germline BRCA1/2 (gBRCA1/2) mutations and have not shown efficacy in homologous recombination DNA repair (HRR) proficient tumors. In pre-clinical models, the BET inhibitor (BETi) ZEN003694 sensitizes wild-type BRCA1/2 tumors to PARPi through downregulation of HRR gene expression, providing a rationale for combination therapy. We report initial results from a Ph 1b/2 trial evaluating the combination of ZEN003694 and the PARPi, talazoparib, in TNBC patients without gBRCA1/2 mutations. Methods: A Ph 1b dose-finding segment will be followed by a single-arm Ph 2 Simon 2-stage segment. Ph 1b evaluated several dose combinations of ZEN003694 and talazoparib, with safety and recommended Ph 2 dose (RP2D) as primary endpoints and pharmacokinetics (PK), pharmacodynamics (PD), and clinical benefit rate (CBR = Objective response rate (ORR) + stable disease > 4 months) as secondary endpoints. The Ph 2 segment has CBR as the primary endpoint and progression free survival (PFS) and duration of response as secondary endpoints. Eligibility criteria included TNBC (ER/PR < 10% and not a candidate for endocrine therapy), HER2-, wild-type gBRCA1/2, and > 1 prior chemotherapy regimen for metastatic disease. Patients were dosed daily in continuous 28-day cycles until disease progression or unacceptable toxicity. Dose limiting toxicity (DLT) period was one cycle. Adverse events (AE), PK, and PD in whole blood and tissue biopsies were assessed. Response endpoints were assessed per RECIST 1.1 every 2 cycles. Results: Findings of the Ph 1b are reported. 15 patients with a median 3 lines of prior therapy in the metastatic setting were enrolled in 3 dose-finding cohorts. RP2D was determined to be 48mg ZEN003694 plus 0.75mg talazoparib. Across the cohorts, the most common AE was thrombocytopenia (TCP) (73%) with 53% G3/4 (Table 1). G4 TCP was the DLT and 1 DLT patient required a platelet transfusion. TCP could be managed to G1/2 levels with intermittent dose holds and reductions. Other G1/2 AEs included fatigue, anorexia, neutropenia, nausea, dysgeusia, and photophobia. Dose intensity analysis showed average daily doses of ZEN003694 and talazoparib could be maintained above 40mg and 0.5mg, respectively, over 4 cycles. Exposures of ZEN003694 and talazoparib were dose proportional with no drug-drug PK interactions. At RP2D, PD assessment by a whole blood mRNA assay for BET-dependent genes demonstrated robust down-regulation of CCR1, IL1RN, and IL8 to < 50% of baseline for > 8 h. Expression of HRR genes, RAD51 and BRCA1, in whole blood also decreased for > 8 h. Analysis of an on-treatment biopsy showed robust and durable BET target modulation assessed by comparing RNA sequence data with a reference BET dependent signature. Across the 3 cohorts, ORR by Investigator was 38% (5/13), including 1 CR and 4 PRs, and CBR was 57% (8/14). 6 of the 15 patients are ongoing as of data analysis date (2-9 cycles), with 1 patient responding for > 6 months. Conclusions: Combination of ZEN003694 and talazoparib demonstrated anti-cancer activity in pretreated metastatic TNBC patients without gBRCA1/2 mutations. TCP is frequent but manageable with dose adjustments. PK is predictable, and PD data show meaningful target engagement. The Ph 2 part of the trial is currently ongoing. Grade 3/4 Adverse EventsCohort 1(1mg talazoparib + 48mg ZEN003694)N=6Cohort 2(0.75 mg talazoparib + 48mg ZEN003694)N=6Cohort 3(1mg talazoparib + 36mg ZEN003694)N=3Thrombocytopenia3 (G3), 2 (G4, DLT)1 (G3), 1 (G4, DLT)1 (G3)Diarrhea1 (G3)00Neutropenia01 (G3)0 Citation Format: Philippe Aftimos, Mafalda Oliveira, Kevin Punie, Valentina Boni, Erika Hamilton, Ayca Gucalp, Payal Shah, Lida Mina, Priyanka Sharma, Lisa Bauman, Eric Campeau, Sarah Attwell, Margo Snyder, Karen Norek, Akos Czibere, Yanke Yu, Michael H Silverman, Sanjay Lakhotia, Susan Domchek, Jennifer Litton, Mark Robson. A Phase 1b/2 Study of the BET inhibitor ZEN003694 in combination with talazoparib for treatment of patients with TNBC without gBRCA1/2 mutations [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 PS11-10.

Chemotherapy-induced peripheral neuropathy is a dose-limiting side effect of many antineoplastic agents, but the mechanisms underlying the toxicities are unclear. At their MTDs, the microtubule-binding drugs paclitaxel and ixabepilone induce more severe neuropathy in mice relative to eribulin mesylate, paralleling their toxicity profiles in clinic. We hypothesized that the severity of their neurotoxic effects might be explained by the levels at which they accumulate in the peripheral nervous system. To test this hypothesis, we compared their pharmacokinetics and distribution in peripheral nerve tissue. After administration of a single intravenous dose, each drug was rapidly cleared from plasma but all persisted in the dorsal root ganglia (DRG) and sciatic nerve (SN) for up to 72 hours. Focusing on paclitaxel and eribulin, we performed a 2-week MTD-dosing regimen, followed by a determination of drug pharmacokinetics, tissue distribution, and multiple functional measures of peripheral nerve toxicity for 4 weeks. Consistent with the acute dosing study, both drugs persisted in peripheral nervous tissues for weeks, in contrast to their rapid clearance from plasma. Notably, although eribulin exhibited greater DRG and SN penetration than paclitaxel, the neurotoxicity observed functionally was consistently more severe with paclitaxel. Overall, our results argue that sustained exposure of microtubule-binding chemotherapeutic agents in peripheral nerve tissues cannot by itself account for their associated neurotoxicity. Cancer Res; 76(11); 3332–9. ©2016 AACR.

E7389 (eribulin mesylate, Halaven, Eisai) is a fully synthetic analog of the marine sponge natural product halichondrin B. It has been approved in US and more than 50 countries worldwide for treatment of advanced breast cancer. To improve the therapeutic index of E7389, a liposomal formulation of E7389, E7389-LF is under development. In this study, we characterized the pharmacokinetics of E7389-LF and E7389 (aqueous formulation) in mice and evaluated the anti-tumor activity of E7389-LF and E7389 in mouse xenograft model. In general the pharmacokinetics of E7389-LF was characterized by a small volume of distribution, slow clearance, and slow elimination in all liposomal formulations tested. Plasma AUC of E7389-LF was more than 600 fold larger than that of E7389 in tumor bearing mice. Tumor AUC of E7389-LF was also observed. Moreover, free E7389 (not liposome encapsulated and not protein bound) was measured in mouse plasma by ultracentrifugation method. The concentration ratio of E7389 in ultracentrifuged mouse plasma (UCM) vs E7389 in plasma after a 2 mg/kg i.v. of E7389 ranged from 54.19% to 65.41%, which was similar to the free fraction in the mouse plasma. The respective concentration ratio of E7389 in UCM vs E7389 in plasma after a 2 mg/kg i.v. of E7389-LF ranged from 0.07% to 0.59%, and the exposure, expressed as AUC, of UCM/plasma ratio was determined to be 0.2%. The free E7389 exposure, measured by UCM, after E7389-LF administration was about 1.5 fold of that after E7389 administration. Anti-tumor activity of E7389-LF and E7389 was evaluated in a human pharynx carcinoma, FaDu, xenograft model. E7389-LF showed dose-dependent anti-tumor effect with regression of tumors at 2.5 mg/kg dose, which was significantly better than that after E7389 administration in this model. These findings suggest that E7389-LF showed better anti-tumor activity than that of E7389 in human FaDu xenograft mouse model. Citation Format: Makoto Asano, Kenji Hyodo, Yanke Yu, Edgar Schuck, Junji Matsui, Hiroshi Ishihara, Hiroshi Kikuchi, Kenichi Nomoto. Characterization of the liposomal formulation of eribulin mesylate (E7389) in mice. [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 4543. doi:10.1158/1538-7445.AM2015-4543

Neuropathy arising from chemotherapy (CIPN) is a major clinical problem representing the dose-limiting side effect of many antineoplastic drugs. At their respective MTDs, we previously reported paclitaxel and ixabepilone produced more severe deficits in nerve conduction velocity, amplitude and degenerative changes in dorsal root ganglia (DRG) and sciatic nerve (SN) versus eribulin mesylate (Wozniak et al 2011). Similar trends for eribulin to cause less neuropathy have also been reported in the clinic (Cigler and Vadhat 2010, Jain and Cigler 2012, Vadhat et al 2013). The underlying reason for this differential effect remains elusive. Differences in tubulin binding may, in part, explain the differential effect (Perez et al 2009, Jordan et al 2005). Another potential explanation could reside in different pharmacokinetic (PK) and nervous tissue distribution of these agents. Toward this latter point, we conducted tissue distribution and pharmacokinetic studies following acute dosing of eribulin mesylate, paclitaxel and ixabepilone. We reported that while all three drugs rapidly cleared from plasma, they distributed into and cleared slowly (>3 days) from peripheral nervous system tissues (Wozniak et al 2014). We have now extended these studies to look out to 42 days following both acute and 2 week MTD-dosing paradigms evaluating drug pharmacokinetics in plasma, DRG and SN as well as pharmacodynamic measurements of nerve conduction and amplitude. We found plasma levels of eribulin mesylate and paclitaxel rapidly declined over 24h after infusion. In contrast, the levels of eribulin mesylate and paclitaxel persisted at above quantification levels (ranging from 5-10 ng/g) for 7 to 14 days post cessation of 2 week MTD dosing. Eribulin mesylate consistently showed greater distribution into DRG and SN over paclitaxel. When comparing the induced nerve conduction deficits, we found paclitaxel and eribulin mesylate caused reduction of nerve conduction parameters similar to our previous studies, most notably in caudal nerve amplitude (by 80.5% and 54.9% respectively). However these deficits, which were sustained for up to several weeks after cessation of dosing, were more severe with paclitaxel. The findings demonstrate that despite eribulin mesylate showing higher nervous tissue distribution than paclitaxel, eribulin mesylate caused less neurotoxicity as evidenced by milder reductions in nerve conduction and amplitude. This work was financially supported by Eisai. Citation Format: Krystyna Wozniak, Ying Wu, Bruce A. Littlefield, Kenichi Nomoto, Christopher DesJardins, Yanke Yu, George Lai, Larisa Reyderman, Barbara S. Slusher. Pharmacokinetics and pharmacodynamic analysis of eribulin mesylate and paclitaxel in mouse. [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 4503. doi:10.1158/1538-7445.AM2015-4503

Sulforaphane, a Dietary Component of Broccoli/Broccoli Sprouts, Inhibits Breast Cancer Stem Cells The existence of cancer stem cells (CSCs) in breast cancer has profound implications for cancer prevention. The purpose of this study was to evaluate sulforaphane, a naturally occurring dietary compound from broccoli/broccoli sprouts, for its efficacy to inhibit breast CSCs as well as to elucidate the potential mechanisms of its effect. Aldefluor and mammosphere formation assays were used to assess the effect of sulforaphane on breast CSCs in vitro. A NOD/SCID xenograft model was employed to determine the efficacy of sulforaphane in targeting breast CSCs in vivo. The potential mechanism of sulforaphane was investigated utilizing Western blotting and β-catenin reporter assays. The results showed that sulforaphane (1∼5 µM) decreased aldehyde dehydrogenase (ALDH)-positive cell population by 65%∼80% in human breast cancer cell line (P < 0.01), and reduced the size and number of primary mammospheres by 8∼125-fold and 50%∼75% (P < 0.01), respectively. Daily injection with 50 mg/kg sulforaphane for two weeks reduced ALDH-positive cells by more than 50% in tumors (P = 0.003). Furthermore, sulforaphane eliminated breast CSCs in vivo, thereby abrogating tumor re-growth in secondary mice (P < 0.01). Western blotting showed that sulforaphane down-regulated the Wnt/β-catenin self-renewal pathway. These data suggest that sulforaphane targets breast CSCs through the down-regulation of the Wnt/β-catenin pathway as a potential mechanism. These findings provide a scientific rationale for use of sulforaphane and potentially broccoli/broccoli sprout dietary interventions for breast cancer chemoprevention. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4271.

The small heat shock protein 27 (Hsp27) is a molecular chaperone that is involved in a variety of cellular functions in cancer cells. The purpose of this research is to study the function of Hsp27 in metastasis of head and neck squamous cancer cells. The expression of Hsp27 in primary and metastatic head and neck cancer cells was determined using real-time PCR and western blotting. The siRNA knockdown of Hsp27 was performed in metastatic head and neck cancer cells using in vitro transfection. The proliferation of the primary and metastatic head and neck cancer cells were evaluated using MTS proliferation assay. The metastatic potential was determined using both migration assay and invasion assay in metastatic head and neck cancer cells after siRNA silencing of Hsp27. MTS proliferation showed that both primary (UM-SCC-22A) and metastatic (UM-SCC-22B) head and neck cancer cells have similar proliferation rate from 24-48 hrs. However, the metastatic head and neck cancer cells (UM-SCC-22B) showed more than 2-fold migration ability and more than 3-fold invasion ability than primary head and neck cancer cells (UM-SCC-22A). Real-time PCR demonstrated that Hsp27 mRNA is 22-fold higher in metastatic UM-SCC-22B than primary UM-SCC-22A. Similarly, Western blotting exhibited that primary UM-SCC-22A did not show any detectable levels of Hsp27, while metastatic UM-SCC-22B showed significant high levels (> 40-fold) of Hsp27. siRNA of Hsp27 knocked down expression of Hsp27 by more than 5-fold in mRNA level and 23-fold in protein levels in metastatic head and neck cancer cells UM-SCC-22B. Furthermore, siRNA knockdown of Hsp27 decreased metastasis of UM-SCC-22B by more than 50% in cell invasion assay. These data suggest that Hsp27 may regulate metastatic potential of head and neck squamous cancer cells. Silencing Hsp27 may decrease metastatic potential of head and neck squamous cancer cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3392.

The purpose of this study is to investigate Hsp90/Cdc37 interaction and identify residues critical for the interaction using real-time luciferase complementation imaging and computational modeling. Molecular imaging using a split Renilla luciferase fragment complementation (SRL-FC) was established to monitor real-time Hsp90/Cdc37interactions in living cells. Computational modeling and molecular dynamics simulations were used to evaluate the interaction interface of Hsp90/Cdc37 complex and critical residues in the interaction interface were identified for mutagenesis. Western Blotting and co-immunoprecipitation were used to determine protein expression and complex formation. The molecular imaging in living cells showed that Hsp90 and Cdc37 helped the complementation of N-terminus of Renilla luciferase (fused with Hsp90; N-RL-Hsp90) and C-terminus of Renilla luciferase (fused with Cdc37, Cdc37-C-RL) with 170-fold higher luciferase activity than controls without Hsp90/Cdc37. This suggests that the SRL-FC is sensitive and specific to monitor the interaction of full length human Hsp90 and Cdc37. Molecular dynamics simulations revealed that the interface was constituted by a series of residues interacting through hydrophobic or polar interactions. Mutagenesis confirmed that mutations in Hsp90 (A121N, Q133A, F134A) and mutations in Cdc37 (M164K, R167A) led to the loss of complementation of N-RL-Hsp90 and Cdc37-C-RL (5 to 10-fold lower luciferase activity compared to wild-type), indicating these residues are critical in Hsp90/Cdc37 interactions. In comparison, mutations in Hsp90 (E47A, S113A) and mutations in Cdc37 (A204E) only decreased 50% of the complementation of N-RL-Hsp90 and Cdc37-C-RL, which suggests that these residues contribute less to Hsp90/Cdc37 interaction. In contrast, mutations of Hsp90 (R46A, S50A) and mutation in Cdc37 (L165H) did not change the complementation of N-RL-Hsp90 and Cdc37-C-RL, indicating that these residues did not contribute to Hsp90/Cdc37 interaction. The data suggests that molecular imaging using split Renilla luciferase fragment complementation (SRL-FC) can be used to identify the critical residues in Hsp90/Cdc37 interaction. Although Hsp90/Cdc37 interactions are through both hydrophobic and polar interactions, mutation in a single amino acid residue, including Ala121, Gln133, Phe134 in human Hsp90 and Met164, Arg167 in human Cdc37, is sufficient to disrupt Hsp90/Cdc37 interaction. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2672.