The Role of Hsp90 in Retinal Proteostasis and Disease

Photoreceptors are sensitive neuronal cells with great metabolic demands, as they are responsible for carrying out visual phototransduction, a complex and multistep process that requires the exquisite coordination of a large number of signalling protein components. Therefore, the viability of photoreceptors relies on mechanisms that ensure a well-balanced and functional proteome that maintains the protein homeostasis, or proteostasis, of the cell. This review explores how the different isoforms of Hsp90, including the cytosolic Hsp90α/β, the mitochondrial TRAP1, and the ER-specific GRP94, are involved in the different proteostatic mechanisms of photoreceptors, and elaborates on Hsp90 function when retinal homeostasis is disturbed. In addition, several studies have shown that chemical manipulation of Hsp90 has significant consequences, both in healthy and degenerating retinae, and this can be partially attributed to the fact that Hsp90 interacts with important photoreceptor-associated client proteins. Here, the interaction of Hsp90 with the retina-specific client proteins PDE6 and GRK1 will be further discussed, providing additional insights for the role of Hsp90 in retinal disease.

Phase I dose-escalation study of the HSP90 inhibitor AUY922 in Japanese patients with advanced solid tumors

PURPOSE

AUY922 is a potent non-geldanamycin inhibitor of heat-shock protein 90. This study was carried out in Japanese patients to determine the maximum tolerated dose (MTD), and to characterize safety, tolerability and pharmacokinetics of single-agent AUY922.

METHODS

Japanese patients with advanced solid tumors whose disease had progressed on at least one line of standard therapy, or for whom no standard therapy existed, were treated with AUY922 (intravenous, once-weekly, 28-day cycle, starting dose 8 mg/m(2)).

RESULTS

Thirty-one patients were treated. Two DLTs were reported in one patient of the 54 mg/m(2) cohort; fatigue and decreased appetite (both Grade 3, resolving to Grade 1 within 8 days). No MTD was determined, and the dose recommended for Phase II studies was determined to be 70 mg/m(2) once-weekly. Most common drug-related toxicities were diarrhea, night blindness and nausea. Grade 1 and 2 visual toxicities at high AUY922 doses ≥22 mg/m(2) were observed. Ten patients (32 %) achieved a best overall response of stable disease, and one patient (3 %) achieved a confirmed partial response.

CONCLUSION

Overall, AUY922 exhibited acceptable toxicities and demonstrated potential clinical activity in Japanese patients, with similar safety and pharmacokinetic profiles to those reported in a preceding global Phase I study in Western patients (CAUY922A2101).

A rat retinal damage model predicts for potential clinical visual disturbances induced by Hsp90 inhibitors

In human trials certain heat shock protein 90 (Hsp90) inhibitors, including 17-DMAG and NVP-AUY922, have caused visual disorders indicative of retinal dysfunction; others such as 17-AAG and ganetespib have not. To understand these safety profile differences we evaluated histopathological changes and exposure profiles of four Hsp90 inhibitors, with or without clinical reports of adverse ocular effects, using a rat retinal model. Retinal morphology, Hsp70 expression (a surrogate marker of Hsp90 inhibition), apoptotic induction and pharmacokinetic drug exposure analysis were examined in rats treated with the ansamycins 17-DMAG and 17-AAG, or with the second-generation compounds NVP-AUY922 and ganetespib. Both 17-DMAG and NVP-AUY922 induced strong yet restricted retinal Hsp70 up-regulation and promoted marked photoreceptor cell death 24h after the final dose. In contrast, neither 17-AAG nor ganetespib elicited photoreceptor injury. When the relationship between drug distribution and photoreceptor degeneration was examined, 17-DMAG and NVP-AUY922 showed substantial retinal accumulation, with high retina/plasma (R/P) ratios and slow elimination rates, such that 51% of 17-DMAG and 65% of NVP-AUY922 present at 30 min post-injection were retained in the retina 6h post-dose. For 17-AAG and ganetespib, retinal elimination was rapid (90% and 70% of drugs eliminated from the retina at 6h, respectively) which correlated with lower R/P ratios. These findings indicate that prolonged inhibition of Hsp90 activity in the eye results in photoreceptor cell death. Moreover, the results suggest that the retina/plasma exposure ratio and retinal elimination rate profiles of Hsp90 inhibitors, irrespective of their chemical class, may predict for ocular toxicity potential.

First-in-human phase I dose-escalation study of the HSP90 inhibitor AUY922 in patients with advanced solid tumors

PURPOSE

A phase I study was conducted with the primary objective of determining the maximum tolerated dose (MTD) of AUY922 in patients with advanced solid tumors. Secondary objectives included characterization of the safety, pharmacokinetic, and pharmacodynamic profiles.

PATIENTS AND METHODS

Patients with advanced solid tumors received 1-hour i.v. infusions of AUY922 once a week in a 28-day cycle. An adaptive Bayesian logistic regression model that employed observed dose-limiting toxicities (DLT) in the first treatment cycle was used to guide dose-escalation decisions, with the established MTD to be used in phase II studies.

RESULTS

One hundred and one patients were enrolled and explored at doses in the range of 2 to 70 mg/m(2). DLTs occurred in 8 patients (22-70 mg/m(2)) and included diarrhea, asthenia/fatigue, anorexia, atrial flutter, and visual symptoms. At 70 mg/m(2), the AUY922 concentration achieved was consistent with active concentrations in a range of xenograft models. There was evidence of target inhibition in peripheral blood mononuclear cells (HSP70 induction) and tumor (client protein depletion and reduction of metabolic activity by (18)F-FDG PET). The recommended phase II dose (RP2D) of 70 mg/m(2) was proposed on the basis of toxicity and pharmacokinetic and pharmacodynamic profiles.

CONCLUSIONS

At the RP2D of 70 mg/m(2), AUY922 exhibited acceptable tolerability, and phase II single-agent and combination studies have been initiated in patients with HER2-positive breast, gastric, and non-small cell lung cancers.

Ocular Toxicity of Targeted Therapies

Molecularly targeted agents are commonly used in oncology practice, and many new targeted agents are currently being tested in clinical trials. Although these agents are thought to be more specific and less toxic then traditional cytotoxic chemotherapy, they are associated with a variety of toxicities, including ocular toxicity. Many of the molecules targeted by anticancer agents are also expressed in ocular tissues. We reviewed the literature for described ocular toxicities associated with both approved and investigational molecularly targeted agents. Ocular toxicity has been described with numerous approved targeted agents and also seems to be associated with several classes of agents currently being tested in early-phase clinical trials. We discuss the proposed pathogenesis, monitoring guidelines, and management recommendations. It is important for oncologists to be aware of the potential for ocular toxicity, with prompt recognition of symptoms that require referral to an ophthalmologist. Ongoing collaboration between oncologists and ocular disease specialists is critical as the use of molecularly targeted agents continues to expand and novel targeted drug combinations are developed.