Abstract 1554:  IpY.20, a first-in-class inhibitor of p27Kip1, inhibits CDK4/6 and CDK2 to kill tumor cells
Authors: Chen Grace , Stafford Lena , Aditya Anusha , Allamneni Krishna , Zisman Natalia , Rashidzadeh Hassan , Blain Stacy W. 
Published: June 2023

Abstract
The drug resistance seen in the presence of CDK4/6 inhibitors, such as palbociclib, can be attributed to compensatory activation of tumor cell proliferation by CDK2. While combination therapy of CDK4/6i with Estrogen modulators significantly extends Progression Free Survival (PFS) in patients with metastatic HR+ breast cancer, their tumors invariably develop resistance, which results in only marginal improvement in their Overall Survival (OS). To address this unmet medical need, both CDK4/6 and CDK2 must be inhibited simultaneously.
p27Kip1, the natural inhibitor of CDK4/6 and CDK2, is responsible for turning them ON and OFF during periods of proliferation and quiescence, respectively. The transition from ON to OFF state is mediated by a specific modification to p27 itself, by the tyrosine kinase BRK (breast tumor related kinase). A naturally-occurring ALTernatively spliced form of BRK, named ALT-BRK or ALT, can bind to p27, block BRK's association and prevent BRK's phosphorylation of p27. The binding of ALT to BRK locks both CDK4/6 and CDK2 into the OFF conformation and simultaneously inhibits activities of both kinases.
We engineered IpY.20, a smaller and more stable variant of ALT that is formulated within a liposome, to increase its serum residence time and facilitate delivery into tumor cells. IpY.20 inhibits cell proliferation of HR+ and triple negative (TN) breast cancer, ovarian, uterine and NSCLC, preferentially over non-tumor cells, supporting its clinical utility. Unlike CDK4/6 and CDK2i which only cause cytostasis, IpY.20 arrests proliferation of CDK4/6i-resistant and endocrine-resistant BC cells and induces cell death, demonstrating that p27 inhibition is a viable therapeutic approach to combat drug resistance due to resurgence of tumor cell growth due to escape from quiescence. In vivo, IpY.20 reduces tumor volume and increases OS in cell line-derived xenografts (CDX).
IpY.20 has several important differentiating features when compared to CDK4/6 or CDK2 inhibitors: targeting p27 is more selective with fewer off-target effects than the ATP-competitive small molecule CDK4/6i currently in use; IpY.20 induces tumor cell death as opposed to cytostasis; the specific form of tumor cell death that is induced upon IpY.20 treatment, i.e., necroptosis, stimulates the innate and adaptive immune system.
As the bulk of drug resistance seen with CDK or RAS/MAPK pathway targeting drugs is a result of compensatory CDK2 activity, IpY.20 will be a valuable member of the clinical arsenal for patients with drug resistant tumors. Currently, a robust clinical manufacturing process has been completed for IpY.20 formulation; along with a comprehensive characterization of nonclinical pharmacology, PK/PD, and tolerability, in preparation for the initiation of an IND-enabling program.
Citation Format: Grace Chen, Lena Stafford, Anusha Aditya, Krishna Allamneni, Natalia Zisman, Hassan Rashidzadeh, Stacy W. Blain.  IpY.20, a first-in-class inhibitor of p27Kip1, inhibits CDK4/6 and CDK2 to kill tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1554.