Abstract 1619: Hsp90 inhibitor drug conjugates (HDCs): Construct design and preliminary evaluation

Background: Most cytotoxic agents are often broadly active but non-selective, and have the disadvantage of high toxicity due to collateral damage to normal tissues. Drugs that target specific protein drivers of cancer cell growth are more tumor selective, yet often lead to tumor resistance via point mutations in their target or activation of alternative signaling pathways. Targeted delivery strategies, such as Antibody-Drug Conjugates (ADCs), offer a solution to these limitations by delivering potent anti-cancer payloads more directly to tumors. Hsp90 is a chaperone protein required by many cancer cells to maintain the stability and function of numerous proteins that drive cancer cell growth, survival, and metastasis. Small molecule inhibitors of Hsp90 such as ganetespib, PU-H71 and 17-AAG are found to be retained in tumors with half- time up to 65 hours in mouse xenografts. These properties are believed to be due to overexpression of an active form of Hsp90 in cancer cells as compared to normal tissues, and have recently been applied for tumor imaging in patients. Results: We have developed a small molecule drug conjugate platform technology using the unique properties of Hsp90 proteins and Hsp90 inhibitors. We have synthesized HDCs with various Hsp90 inhibitor scaffolds including resorcinol, purine, geldanamycin, etc. A cell-based client protein degradation assay is carried out to confirm the intracellular uptake and the Hsp90 binding of the conjugates. While different Hsp90 inhibitor scaffolds offer different DMPK and toxicology profiles, cellular uptake and Hsp90 binding are not greatly affected by the different chemical classes of Hsp90 inhibitors. Over 30 payloads have been conjugated with Hsp90 inhibitors so far. We have selected the payloads based on the hypothesis that HDC can improve the safety profile of a cytotoxic drug, expand the application of chemotherapeutic agents to different tumor types, combat drug resistance, and enable novel anticancer approach. Examples of payloads include topoisomerase inhibitors (camptothecin), microtubule modulators (taxanes), proteasome inhibitors (carfilzomib), CDK inhibitors (flavopiridol) and others. Unlike most other conjugate technologies, HDC does not require lengthy spacing between the anchor Hsp90 inhibitor and the payload. Linker cleavage mechanism is considered a key feature in the HDC design. We have been able to incorporate several linkers such as disulfide, hydrazone, peptide, carbamate, carboxylate