Project 3 - Development of serine/threonine phosphatase PP2A molecular glues for the treatment of advanced prostate cancer

University Of Michigan At Ann Arbor, Ann Arbor MI

Investigators

Linked publications & trials

Abstract

Protein Phosphatase 2A (PP2A) functions as a tumor suppressor and is frequently inactivated in cancer by genetic and non-genetic mechanisms. PP2A is a serine/threonine-specific phosphatase and is a heterotrimeric complex composed of a scaffolding subunit (A), regulatory subunit (B), and catalytic subunit (C). The ability of PP2A to dephosphorylate a vast repertoire of distinct substrates is regulated by over 40 specificity determining regulatory B subunits that compete for assembly of heterogenous PP2A heterotrimers, resulting in exquisite substrate specificity of the fully assembled, active PP2A heterotrimer. Resulting from its obligate heterotrimeric structure, PP2A biogenesis is highly regulated through predominantly non-genetic mechanisms, specifically carboxymethylation of the terminal L309 residue catalyzed by the leucine carboxy methyltransferase (LCMT1). The presence or absence of carboxymethylation dictates the binding of regulatory B subunits through the neutralization of the negative charge on the carboxy-terminus of the C subunit tail. In general, carboxymethylation of L309 is required for the formation of tumor suppressive PP2A holoenzymes, and loss of this post-translational modification is commonly seen in cancer. We recently showed that loss of PP2A carboxymethylation is a common event in prostate cancer (PCa) and is associated with progression, increased metastatic potential, and treatment resistance to androgen deprivation therapies. Genetic depletion of LCMT1 results in AR and MYC activation and drives PCa growth. We and others have demonstrated that, specifically, the PP2A AB56αC heterotrimer is a major negative regulator of AR and MYC signaling. We have developed a series of first-in-class PP2A molecular glues (PMGs) that stabilize the tumor suppressive PP2A AB56αC heterotrimer, resulting in inhibition of PCa growth both in vitro and in vivo. The lead compound, RPT04402, has undergone extensive preclinical toxicology studies in two animal species (rat and dog) and has entered IND-enabling studies with the goal of filing an IND in November 2024. The goals of this proposal are to define the depth and breadth of activity of RPT04402 in translationally aligned xenograft and patient-derived xenograft models of metastatic castration-resistant prostate cancer, identify substrate/ effector pathways that drive preclinical efficacy, and evaluate RPT04402 as a monotherapy and, if preclinical data warrant, in combinations (e.g., enzalutamide) through a clinical trial at the University of Michigan and Karmanos Cancer Institute (collaborating site). The three Specific Aims of this project are directed at (1) Elucidating the mechanism of action of PP2A PMGs in regulating AR signaling, (2) Conducting comprehensive efficacy studies of the lead PMG candidate alone and in combination with AR targeting agents in preclinical models, and (3) Determining predictive biomarkers of response to PMG monotherapy through a phase I/II trial. We anticipate the proposed studies will provide new and important insights into the role of PP2A in PCa biology, and the results of this project will impact the development of PP2A-based therapeutics for the treatment of metastatic PCa.