A Phase 1 study of PF-04929113 (SNX-5422), an orally bioavailable heat shock protein 90 inhibitor in patients with refractory solid tumor malignancies and lymphomas

Abstract

Purpose

To determine the maximum tolerated dose (MTD), toxicities, and pharmacokinetic-pharmacodynamic profile of the heat shock protein 90 (Hsp90) inhibitor PF-04929113 (SNX-5422) in patients with advanced solid tumors and lymphomas.

Methods

This was a single institution, phase I, dose-escalation study of PF-04929113 dosed twice-weekly. Endpoints included determination of dose-limiting toxicities (DLT), MTD, the safety profile of PF-04929113, pharmacodynamic assessment of PF-04929113 on Hsp70 induction, pharmacokinetic (PK) analysis of PF 04928473 (SNX-2112) and its prodrug PF-04929113 and assessment of response.

Results

Thirty three patients with advanced malignancies were treated. Dose escalation was continued up to 177 mg/m² administered orally twice a week. One DLT (non-septic arthritis) was noted. No grade 4 adverse events (AEs) were seen; grade 3 AEs included diarrhea (9%), non-septic arthritis (3%), AST elevation (3%) and thrombocytopenia (3%). No objective responses were seen in 32 evaluable patients. Fifteen patients (47%) had stable disease; 17 patients (53%) had progressive disease. PK data revealed rapid absorption, hepatic and extra-hepatic clearance, extensive tissue binding and almost linear pharmacokinetics of the active drug PF 04928473. PD studies confirmed inhibition of Hsp90 and a linear correlation between PK parameters and Hsp70 induction.

Conclusions

PF-04929113 administered orally twice weekly is well tolerated and inhibits its intended target Hsp90. No objective responses were seen but long lasting stabilizations were obtained. Although no clinically significant drug-related ocular toxicity was seen in this study the development of PF-04929113 has been discontinued due to ocular toxicity seen in animal models and in a separate phase I study.

Introduction

Heat-shock protein 90 (HSP90) belongs to a family of highly conserved proteins that play an integral role within cells acting as molecular chaperones to numerous biologically important client proteins essential for constitutive cell signaling and adaptive response to stress.(1, 2) Cancer cells use the Hsp90 chaperone machinery to protect an array of mutated and over-expressed oncoproteins from misfolding and degradation.(3) Many potential partner proteins in the Hsp90 interactome have been identified including protein kinases (e.g. EGFR, HER2, Src, Akt, BRAF, BCR-ABL), steroid receptors (e.g. ER, PR, AR), telomerase reverse transcriptase, nitric oxide synthase, and transcription factors (e.g. HIF1α, aryl hydrocarbon receptor, p53, STAT3), many of which are dysregulated in human cancer.(4) Preclinical data show that Hsp90 inhibitors can abrogate the oncogenic switch which is frequently induced as a resistance mechanism to tyrosine kinase inhibitors (TKIs). Interestingly most of the induced and/or mutated kinases that have been identified (HER2, BRAF, MET and ALK) are Hsp90 clients and remain sensitive to Hsp90 inhibition.(5) Similarly the development of secondary mutations e.g. T790M that provide resistance to EGFR TKIs also remain sensitive to Hsp90 inhibition.(6, 7) Targeting Hsp90 is potentially a powerful strategy in cancer therapeutics due to the central role this protein plays in many simultaneous oncological signaling pathways.(8)

PF-04929113 (SNX-5422) is a water soluble and orally bioavailable prodrug of PF 04928473 (SNX-2112) a potent and highly selective small-molecule inhibitor of Hsp90.(9, 10) PF 04928473 competitively binds to the N-terminal adenosine triphosphate (ATP) pocket of Hsp90 family members (Hsp90α, Hsp90β, Grp94, and Trap-1) and is highly potent against various cancers in vitro and in vivo.(9–11) Based on these results a single institution phase I study was conducted to evaluate the maximum tolerated dose (MTD) and safety profile of PF-04929113 when administered twice a week every 28 days using a continuous dosing schedule.

Patients and Methods

Patients

Eligibility criteria included: histologically documented solid tumors and lymphoid malignancies (lymphoma and CLL) refractory to or for whom there is no standard therapy, measurable or evaluable disease, age > 18 years, ECOG performance status < 2, life expectancy > 3 months, adequate organ and bone marrow function, and the ability to understand and willingness to sign informed consent. Patients were not permitted to have major surgery, radiation therapy, chemotherapy or biologic therapy within 4 weeks prior to entering the study and any toxicity related to previous therapy should have recovered to at least grade 1. Patients with symptomatic brain metastases or HIV infection on anti-retroviral therapy were also excluded.

The primary endpoints for this phase I study were to determine the MTD, safety, and toxicity of PF-04929113 when administered twice a week for 28 days. Secondary objectives included: investigation of the effects of PF-04929113 on engagement of the Hsp90 target by pharmacodynamic assessment of Hsp70 levels, assessment of response using the National Cancer Institute (NCI) Response Evaluation Criteria in Solid Tumors (RECIST 1.0) criteria for solid tumors(12) and standardized lymphoma criteria(13) for lymphomas, and determining the pharmacokinetic (PK) profile of the active drug PF-04928473 (SNX-2112) in humans.

Study Design

Cohorts of 3–6 patients were enrolled at each dose level. The dose level at which 2 patients experienced dose limiting toxicity (DLT) was considered to have exceeded the MTD. The next lower dose level at which no more than 1/6 patients experienced DLTs was considered the MTD. DLTs were defined as adverse events possibly, probably or definitely related to administration of PF-04929113 and fulfilling any of the following criteria: grade 4 non-hematological and hematological toxicities with the exception of grade 4 neutropenia lasting < 5 days without fever or infection; and grade 3 non-hematological toxicities with the following exceptions: grade 3 nausea, vomiting, diarrhea and electrolyte abnormalities if refractory to treatment, grade 3 creatinine if not correctable to grade 1 or less after 2 liters of intravenous fluids within 48 hours, and grade 3 elevation in liver transaminases and/or bilirubin if they did not return to baseline after a maximum of two dropped doses (not to exceed 10 days) within a cycle or 15 days between cycles.

The starting dose of PF-04929113 was 4 mg/m² per dose. The dose escalation schema and the number of patients treated is depicted in Supplemental Table 1. Intra-patient dose escalation was permitted per protocol if: a) there was no drug-related toxicity >Grade 1 after one course at the initial dose level, b) the higher dose level had been completed by all patients in that cohort and no patients experienced a DLT, and c) disease had not progressed. Patients self-administered the assigned dose of PF-04929113 with 225 mL of water twice a week continuously in a 28-day cycle. Food was held 2 hours prior to and 1 hour after ingestion of the study drug. Treatment was continued until disease progression or development of intolerable toxicity. Dose modifications were performed if patients developed severe toxicities based on predefined criteria, according to the Common Terminology Criteria for Adverse Events (CTCAE) version 3.0. Intra-patient dose escalation up to the last evaluated dose not associated with a DLT was permitted if there was no drug-related toxicity >grade 1 after one cycle at the initial dose level, the higher dose level had been completed by all patients in that cohort with no DLTs observed and there was no evidence of disease progression.

Studies performed for baseline assessment and on-study evaluation are presented in the supplemental section. On-study evaluation included an EKG performed on day 1 of each cycle starting with cycle 2. To examine the relationship between plasma concentrations of PF-04929113 and its effect on the corrected QT (QTc) interval, the study was amended in May 2009 to include continuous EKG monitoring beginning one hour pre-dose and continuing for 23 hours post-dose on days 1 and 15 of cycle 1. In July 2010 due to findings from animal studies that showed the potential of PF-04929113 to induce irreversible retinal damage, the study was further amended to include a comprehensive ophthalmologic evaluation including visual acuity and visual field assessment, ophthalmoscopy, dark adaptation testing, Ganzfeld Electroretinography (ERG) and Optical Coherence Tomography (OCT) if these tests hadn’t been performed earlier or if new visual symptoms developed while a patient was on study.

Pharmacokinetic Analysis

Blood was drawn on day 1 and 15/18 of cycle 1 at the following time points: pre-dose, and post-dose at 20 and 40 minutes, and at 1, 2, 3, 4, 6, 8, 10, 12, 24, 36, and 48 hours. During subsequent cycles blood was drawn prior to dosing on day 1 only. Details of sample processing are presented in the supplemental section. Samples were analyzed using a validated LC/MS-MS method.(14) PK parameters, including area under the curve (AUC 0–48), apparent clearance (CL/Fm), volume of distribution (Vz/Fm), maximum concentration (Cmax), time to reach maximum concentration (tmax), and half life for PF-04928473 were calculated by noncompartmental analysis using WinNonlin professional software version 5.0 (Pharsight Corporation, Mountain View, CA, USA).

Pharmacodynamic Analysis

Analysis of Hsp70 induction in peripheral blood was conducted by western blot. Details of sample processing and analysis are presented in the supplemental section.

Statistical Analysis

The dose dependent linearity in PK of PF-04928473 was determined based on slope of the following power model: AUC=intercept • (Dose) ^slope. If 90% CI on slope contained 1, PK was considered linear (i.e., dose-proportional). Statistical comparisons between Hsp70 induction, dose of PF-04929113 and PK parameters were performed using nonparametric Mann-Whitney U test. All tests were two-tailed and statistical analysis software GraphPad Prism v5.0c was used. Spearman correlation analyses were performed using GraphPad Prism software to correlate Hsp70 induction with PK parameters (mean Cmax and AUCinf of each cohort) measured on day 1 of cycle 1. Other secondary evaluations were performed in a descriptive manner. Results from secondary analyses are exploratory in nature, and p-values are presented without correction for multiple comparisons.

Results

Between March 2008 and October 2009, 33 patients were enrolled. Patient characteristics are summarized in Table 1. Intra-patient dose escalation was carried out in four patients: from 4 mg/m² to 16 mg/m² in one patient, from 8 mg/m² to 16 mg/m² in 2 patients, and from 8 mg/m² to 33 mg/m² in 1 patient.

Table 1.

Patient Characteristics

Patient Characteristic	Total
Total number	33
Median age (range), years	57 (38 – 78)
Gender:
Male	17
Female	16
ECOG Performance Status:
1	32
2	1
Race:
Caucasian
- Non-hispanic	23
- Hispanic	3
Black	6
Asian	1
Histology:
Colon Cancer	7
Rectal cancer	2
Esophageal cancer	2
Pancreatic cancer	1
Non-small cell lung cancer	3
Small cell lung cancer	1
Pulmonary carcinoid	1
Peritoneal mesothelioma	2
Ovarian cancer	1
Endometrial cancer	1
Prostate cancer	1
Thyroid cancer	2
Melanoma	1
Lymphoma	3
Chondrosarcoma	1
Bladder cancer	1
Thymic carcinoma	1
Adenoid cystic carcinoma of the salivary gland	1
Adenoid cystic carcinoma of the soft and hard palate	1
Prior systemic therapy	31
Median number of regimens (range)	4 (0 – 8)
Prior radiotherapy	14
Prior surgery	32

Toxicity

PF-04929113 was well tolerated. A DLT was noted in one patient in the form of non-septic arthritis at a dose of 16 mg/m²/dose 18 days after initiation of cycle 3 of treatment. Although this adverse event arose beyond the period for evaluation of toxicity at a given dose level (defined as 28 days after 3 evaluable patients in a dose cohort receive the last dose in cycle 1), the cohort that was enrolling when this adverse event occurred (cohort 5 at 44 mg/m² on which two patients had enrolled) was expanded to six patients and no further DLTs were seen. However, dose escalation was stopped at a dose of 177 mg/m²/dosedue to reports of irreversible retinal damage observed in a canine model after 4 out of 20 patients on another phase I study evaluating PF-04929113 had visual symptoms of nictalopia and blurred vision.

In addition to the grade 3 non-septic arthritis, another patient being treated at a dose of 16 mg/m² experienced joint swelling and stiffness possibly related to the study drug. Further workup in both cases did not reveal any other cause to account for these symptoms.

Most adverse events that could be possibly attributed to the study drug were grade 1 or 2 in severity (Table 2). Grade 3 adverse events possibly related to the drug included diarrhea, non-septic arthritis (described above), AST elevation and thrombocytopenia (Table 3). Grade 3 diarrhea was seen in three patients; two treated at dose level 8 (100 mg/m²) and one patient treated at dose level 10 (177 mg/m²). It was not considered a DLT since it lasted less than 48 hours and was not refractory to treatment. Grade 3 AST elevation possibly related to the investigational agent was seen in one patient at dose level 7 (77 mg/m²). It was not considered a DLT since it resolved within 4 days. Grade 3 thrombocytopenia was seen in one patient at dose level 10 (177 mg/m²).

Table 2.

Adverse events possibly, probably, or definitely attributed to treatment with PF-04929113

Adverse Event	Grade 1	Grade 2	Grade 3	Grade 4
	Number of patients (%)
Nausea	8 (24)	1 (3)	0	0
Emesis	2 (6)	1 (3)	0	0
Anorexia	2 (6)	1 (3)	0	0
Diarrhea	4 (12)	0	3 (9)	0
Constipation	1 (3)	0	0	0
Dysgeusia	1 (3)	1 (3)	0	0
Abdominal pain	1 (3)	0	0	0
Dyspepsia	1 (3)	0	0	0
Weight loss	1 (3)	0	0	0
Fatigue	4 (12)	2 (6)	0	0
Joint pain	1 (3)	1 (3)	0	0
Arthritis (non-septic)	0	1 (3)	1 (3)	0
Muscle pain	0	1 (3)	0	0
Flu-like symptoms	1 (3)	0	0	0
Edema	0	1 (3)	0	0
Cough	1 (3)	0	0	0
AST elevation	3 (9)	2 (6)	1 (3)	0
ALT elevation	5 (15)	1 (3)	0	0
Alkaline phosphatase elevation	0	2 (6)	0	0
Creatinine phosphokinase elevation	1 (3)	0	0	0
Hyponatremia	1 (3)	0	0	0
Hypophosphatemia	0	1 (3)	0	0
Leukopenia	2 (6)	0	0	0
Neutropenia	1 (3)	0	0	0
Lymphopenia	2 (6)	6 (18)	0	0
Anemia	2 (6)	3 (9)	0	0
Thrombocytopenia	4 (12)	0	1 (3)	0

Table 3.

Relationship between grade 3 adverse events at least possibly related to PF-04929113 and dose level

Grade 3 adverse event	Cohort	Dose Level
AST elevation	7	77 mg/m2
Diarrhea	8	100 mg/m2
	8	100 mg/m2
	10	177 mg/m2
Non-septic arthritis	3	16 mg/m2
Thrombocytopenia	10	177 mg/m2

Continuous EKG monitoring did not reveal any effect of PF-04929113 on the corrected QT interval. No drug-related grade 4 adverse events or deaths were seen.

Due to the potential impact of PF-04929113 on visual function, a protocol amendment was implemented requiring patients to undergo a comprehensive ocular exam (see above). Three patients who were actively being treated at dose levels of 77 mg/m², 133 mg/m² and 177 mg/m² were evaluated. One patient requested to come off study after 10 cycles prior to ocular assessment. She was initially treated at 177 mg/m² and subsequently dose reduced to 133 mg/m² due to diarrhea. She complained of blurry vision and was found to have bilateral cataracts. Ocular examination revealed a slightly prolonged cone-rod break in the right eye but the ERG was normal. The two other patients were evaluated after 8 cycles and 28 cycles of treatment and had a normal eye exam.

Duration of Therapy and Responses

Thirty-two patients were evaluable for response: 15 patients (45%) achieved disease stabilization (median duration 16 weeks, range, 3–110 weeks) as best response and 17 patients (51%) progressed on treatment. No objective responses were seen.

The median number of cycles administered was 2 (range 1–28). Three patients received 10 or more cycles. One patient with medullary thyroid cancer received 28 cycles (initial dose level 8 mg/m²). He had received no prior systemic therapy. Another patient with metastatic colon cancer received 11 cycles at 177 mg/m². This patient had received 5 prior lines of systemic therapy. The third patient had metastatic salivary gland adenoid cystic carcinoma and received 10 cycles of treatment (initial dose level 177 mg/m²). This patient had previously received decitabine, depsipeptide and flavopiridol. All three patients had stable disease at the time of discontinuing treatment and the decision to stop treatment was based on data from ongoing animal studies and other phase I studies of the same agent that revealed the potential of PF-04929113 to cause irreversible retinal damage.

Pharmacokinetic Analyses

Plasma concentration – time profiles for PF-04928473 on day 1 and day 15/18 are shown in Figure 1. PF-04929113 is rapidly absorbed following oral administration. Levels of active drug PF-04928473 were detectable at 20 minutes through 48 hours. Supplemental Tables 2 and 3 summarize the descriptive statistics of PK parameters for PF-04928473 on cycle 1, Day 1 and cycle 1, Day 15/18, respectively, across all tested dose levels. Maximum plasma concentrations were normally reached between 1 to 3 hrs. Exposures of PF-04928473 are highly variable with geometric CV% of 8% to 124% on AUC_0–48/AUC_inf and 13% to 133% for C_max. The half lifefor PF-04928473 was almost similar between day 1 and day 15, which ranged from approximately 8 to 15 hrs. The half-life for PF-04928473 doesn’t appear to change across different dose levels. The high clearance value (CL/Fm) indicated that it is both hepatic and extra-hepatic mediated. The high Vz/Fm value indicated extensive tissue binding. There was no drug accumulation after multiple dosing. The observed accumulation ratio for each dose level is listed in Supplemental Table 4, which ranged from 0.65 to 1.83. PF-04928473 followed almost linear PK with slope (90% CI) of 1.23 (1.08–1.38) for power relationship between AUC_inf,ss and dose, and of 1.22 (1.08–1.37) for power relationship between AUC_0–48,ss and dose. Plots of AUC0–48 versus doses for Cycle 1, Day 1 and Cycle 1, Day 15/18 are shown in Supplemental Figures 2 and 3, respectively. These figures show almost dose-proportional increase in AUC_0–48 with increase in dose, consistent with linear pharmacokinetics.

Figure 1.

Mean plasma concentration-time profile for escalating dose levels of PF-04928473 after first dose (semi log scale) on: (A) Cycle 1, Day 1, and (B) Cycle 1, Day 15/18. Error bars are not shown for clarity.

Pharmacodynamic Analyses

Hsp70 level in PBMC

We measured Hsp70 levels by western blot in peripheral blood mononuclear cells (PBMCs) from 27 patients (data not shown). The Hsp70 protein level increased in 16 out of 19 patients at day 15 and/or day 16 (Figure 2) and in 3 out of 4 patients at days 18 and/or 19 compared to baseline. The level of Hsp70 induction was greater at higher dose levels and this was observed at the beginning of each cycle and was sustained mid-cycle, i.e. on day 16 (Figure 3). Interestingly, the degree of Hsp70 induction showed a linear correlation with PK parameters (Cmax and AUCinf of each cohort) as illustrated in Figure 3.

Figure 2.

Hsp70 induction in PBMCs on Day 15/16 after treatment with PF-04929113 compared to baseline.

Figure 3.

Association of Hsp 70 induction with dose of PF-04929113 and PK parameters. (A-C): Hsp70 induction post-dose on Days 1, 15 and 16 as a function of dose (≤ 44 mg/m²/dose vs. > 44 mg/m²/dose). Statistical comparisons were done using the nonparametric Mann-Whitney U test. All tests were two-tailed and statistical analysis software GraphPad Prism v5.0c was used. (D-E) Correlation of Hsp70 induction with PK parameters (mean Cmax and AUCinf of each cohort) measured on day 1 of cycle 1. Spearman correlation analyses were performed using GraphPad Prism software. Natural log of the mean Cmax and AUCinf of each cohort was plotted vs. the fold increase in Hsp70 at 24h versus baseline.

Discussion

This phase I study has shown that treatment with PF-04929113 administered orally twice a week using a continuous dosing schedule is well tolerated. Drug-related AEs were generally mild with the most common being nausea, fatigue and diarrhea. The toxicity profile observed is similar to that reported with other Hsp 90 inhibitors in phase I studies.(15, 16) Schedule of administration of Hsp90 inhibitors has been shown to have a significant effect on drug tolerability.(17–19) PF-04929113 has been evaluated in two separate phase I studies using different schedules of administration; every other day for 21 days of a 28 day cycle and every day for 21 of 28 days continuously (later amended to daily continuous administration) The most frequent adverse events were nausea, emesis, fatigue and diarrhea. DLTs with daily administration included 3 cases of grade 3 diarrhea, 1 case of grade 2 dehydration and grade 3 gastrointestinal hemorrhage and 1 case of grade 3 visual disturbance and grade 3 diarrhea. Four out of 20 patients who were receiving PF-04929113 daily complained of visual symptoms at doses raging from 50 mg/m²/d to 89 mg/m²/d. Three patients complained of nictalopia and one patient complained of blurred vision. These patients had grade 1 visual changes, which were reported as mild darkening of vision. In most cases, the onset of the event was within 2 weeks of beginning the drug, and all cases recovered within a few days from stopping treatment. Although no similar unexplained ocular symptoms were noted in our study, ocular exam did reveal a slightly prolonged cone-rod break in one patient (data not shown). Preclinical data have demonstrated that geldanamycin and 17-AAG have generated cytostaticity and cytotoxicity in cultured human retinal pigment epithelial cells (RPE) which are essential for physiological function of adjacent photoreceptors, possibly by downregulating AKT and ERK1/2- mediated signaling activities.(20) In a phase I study of the Hsp90 inhibitor, AUY922 administered intravenously at doses of 2–70 mg/m² over 1 hour once weekly, night blindness was reported in 19 out of 96 patients treated (20%). Other visual symptoms that were generally grade 1 and 2 and mostly reversible started at 40 mg/m² and included blurred vision, flashing and delayed dark and light accommodation that increased in frequency or severity with dose. Grade 3 darkening of vision at a dose of 70 mg/m² was listed as a DLT in this study.(21) Given these findings we believe it is critical to perform a comprehensive ocular assessment in future trials evaluating Hsp90 inhibitors, since this may represent a class side effect.

Dose escalation of our study was terminated based on the ocular findings in preclinical and clinical studies of this and other HSP90 inhibitors. Further understanding of the ocular toxicity observed in a separate phase I study is warranted in order to continue with the development of PF-04929113. Based on the clinical and preclinical safety observations, and despite the preliminary observation of antitumor activity, PF-04929113 has been withdrawn from clinical testing. PF-04929113 and a structurally different back-up compound show evidence of retinal toxicity by a yet unknown mechanism. These data indicate that HSP90 is a critical component of retinal function and that its prolonged inhibition can lead to irreversible retinal damage with photoreceptor death. Further evaluation is warranted to better understand whether this is a class/target mediated effect of HSP 90 inhibition.

Although no objective tumor responses were seen in our study, 15 out of 32 patients (47%) evaluable for response, achieved disease stabilization. This is worth noting in a cohort of heavily pretreated patients who had progressive disease at enrollment after multiple lines of systemic therapy (median number of prior systemic therapies was 4). Disease stabilization was seen across all dose levels. Supplemental table 5 shows the underlying tumor type in patients who had stable disease on treatment and the duration of disease stabilization in each case.

Our efficacy results are similar to those seen with other Hsp90 inhibitors in patients with advanced solid tumors, where responses are rarely reported.(22–24) In a phase II study of IPI-504 76 patients with NSCLC were enrolled, and 5 patients (7%) had an objective response including 4 with EGFR wild-type tumors and 1 with EGFR mutation. Interestingly among 3 patients with ALK rearrangements, there were 2 PRs and one prolonged SD.(16) These results suggest benefit in patients with certain solid tumors. The next step would be to determine the biologic underpinnings of responses in these patients and potential for patient selection in future trials based on the presence of a tumor that is primarily driven by an Hsp90 client protein.

In our study, pharmacokinetic analysis did not reveal any drug accumulation after administration of multiple doses using a twice weekly schedule. Pharmacodynamic analysis showed induction of Hsp70 in PBMCs. Hsp70 levels increase as a result of Hsp90 inhibitor-induced activation of HSF1, which then enters the nucleus and activates Hsp70 gene expression. An increase in Hsp70 levels was seen in all patients treated at or above a dose of 33 mg/m²/day. Furthermore the increase in Hsp70 levels at 24 hours compared to baseline showed a linear correlation when plotted against the mean Cmax and AUCinf of each cohort (Figure 3). This is the first study that has demonstrated a correlation between PK and Hsp70 induction across dose levels. Since Hsp70 induction is seen at dose levels as low as 33 mg/m²/d, PF-04929113 could be combined at relatively low doses with other systemic therapy in future studies while retaining the ability to inhibit Hsp90. Hsp70 has however been identified as an anti apoptotic protein and it may play a role in HSP90 resistance. Ongoing strategies to overcome this problem include direct inhibition of Hsp70 activity(25, 26), and inhibition of the transcriptional induction of Hsp70 by heat shock factor 1, which has been identified to have potent oncogenic activity(27). The complexity of chaperone biology is further highlighted by reports suggesting that Hsp70 induction may have beneficial neuroprotective activity when an Hsp90 inhibitor is combined with bortezomib(28). One drawback of this study is that PBMCs were used as a surrogate tissue for Hsp70 analysis. The data thus far suggest that Hsp90 exists in a high-affinity state for pharmacologic inhibitors in tumor tissue compared to normal tissues.(29) Coincident with this observation, Hsp90 inhibitors tend to accumulate in tumor tissue as compared to normal tissue.

For the first eight years of clinical development the only Hsp90 inhibitors in clinical trial were the natural product ansamycins, which had major drawbacks in insolubility and formulation. The data reported here is the first full report of a second generation Hsp90 inhibitor that is fully synthetic and orally available.

In conclusion, our study shows that PF-04929113 administered twice weekly using a continuous dosing schedule, is well tolerated with mild adverse events but modest anti-tumor activity when used as a single-agent. The recommended phase 2 dose has not been determined; further evaluation including a better understanding of the mechanism of ocular toxicity, relationship between ocular toxicity and plasma levels and implementation of appropriate testing to evaluate ocular toxicity of PF-04929113 in patients would be needed to define the recommended phase 2 dose. However, based on PD data from our study the lowest biologically effective dose appears to be 33 mg/m² administered orally twice weekly; at or above this dose induction of Hsp70 was observed in all cases. Clearly drug scheduling is an extremely important issue. HSP90 inhibitors given on a daily basis may enhance toxicity where no identifiable clinical benefit can be observed. A deeper understanding of proteins that interact with Hsp90 and further insights gained into molecular aberrations in recurrent or refractory tumors by comprehensive molecular profiling may help in the development of novel combinations of Hsp90 inhibitors such as PF-04929113.

STATEMENT OF TRANSLATIONAL RELEVANCE.

Heat shock protein 90 (Hsp90) serves as a molecular chaperone to multiple oncoproteins and biologically important protein kinases. Although many clinical trials have been performed to evaluate Hsp90 inhibitors, the development of ocular toxicity in animal models as well as clinical trials has raised concerns about further development of these compounds. We believe our study is the first to incorporate comprehensive ophthalmologic evaluation to assess ocular toxicity that is associated with these drugs. Although no clinically significant drug-related ocular toxicity was seen in this study, the occurrence of ocular complaints in a separate phase I study of PF-04929113 using a different schedule of administration suggests an association between schedule of administration and development of ocular toxicity. Unless the cause of these ocular symptoms is determined and resolved satisfactorily, it is likely to have a significant impact on further clinical development of Hsp90 inhibitors.

In addition, to the best of our knowledge this is the first study to describe a linear correlation between a pharmacodynamic endpoint as illustrated by Hsp70 induction and pharmacokinetic parameters across dose levels.

We would be grateful if our manuscript entitled ‘A Phase 1 study of PF-04929113 (SNX-5422), an orally bioavailable heat shock protein 90 (Hsp90) inhibitor in patients with refractory solid tumor malignancies and lymphomas’ could be considered for publication in Clinical Cancer Research.