CB-5083: A Selective p97 Inhibitor for Disrupting Protein Homeostasis in Cancer Research

Introduction

Maintaining protein homeostasis is essential for cellular health, with dysregulation frequently implicated in oncogenesis and tumor progression. The ATPase p97 (valosin-containing protein, VCP) is a central orchestrator of protein quality control, facilitating processes such as organelle membrane fusion, endosomal sorting, and the extraction of misfolded proteins for proteasomal degradation. Targeting p97’s activity has emerged as a promising strategy for cancer therapeutics, particularly in tumors characterized by high proteotoxic stress. Among the p97 inhibitors under investigation, CB-5083 distinguishes itself as a highly selective, orally bioavailable inhibitor, exhibiting robust anti-tumor activity in both in vitro and in vivo settings.

The Mechanism of CB-5083: Selective p97 AAA-ATPase Inhibition

CB-5083 is a small molecule characterized by its molecular weight of 413.47 and a chemical formula of C₂₄H₂₃N₅O₂. Notably, it is insoluble in water but demonstrates high solubility in DMSO (>20.65 mg/mL) and ethanol (>4.4 mg/mL). Mechanistically, CB-5083 exhibits nanomolar potency (IC₅₀ = 15.4 nM against wild-type p97), functioning as a competitive inhibitor of ATP binding within the second ATPase domain (D2) of p97. By selectively targeting this domain, CB-5083 disrupts the protein degradation pathway, leading to the accumulation of poly-ubiquitinated substrates and induction of the unfolded protein response (UPR). This pathway is critical for cancer cells, which often rely on heightened proteostasis to manage increased protein synthesis and mutation-induced misfolding.

Disruption of Protein Homeostasis and Induction of Cancer Cell Apoptosis

Inhibition of p97 by CB-5083 impedes the clearance of misfolded proteins from the endoplasmic reticulum (ER), consequently activating the UPR. Persistent or overwhelming UPR triggers apoptotic pathways, notably via the caspase signaling cascade, ultimately resulting in cancer cell death. Experimental data demonstrate that CB-5083 induces a dose-dependent accumulation of TCRα-GFP in the ER and elevates levels of poly-ubiquitinated proteins in diverse cancer cell lines, such as HEK293T, A549, and HCT116. These effects are tightly linked to apoptosis induction, providing a mechanistic rationale for the anti-tumor efficacy observed in preclinical models.

CB-5083 in Tumor Growth Inhibition: In Vivo Xenograft Models

The translational potential of CB-5083 as an oral bioavailable p97 inhibitor has been validated in several mouse xenograft models. Oral administration of CB-5083 has resulted in significant tumor growth inhibition (TGI), with reductions up to 63% observed in models of colorectal adenocarcinoma, non-small-cell lung cancer, and multiple myeloma. Importantly, these anti-tumor effects are attributed to the disruption of protein homeostasis and the induction of apoptosis rather than non-specific cytotoxicity, as evidenced by the selective accumulation of stress markers and activation of the caspase pathway within tumor tissues. These findings have justified the advancement of CB-5083 into phase 1 clinical trials for both multiple myeloma and solid tumors, where its safety and efficacy profiles are under active investigation.

Expanding the Landscape: Intersections with ER Lipid Homeostasis

Recent research highlights the intricate relationship between protein quality control and ER lipid metabolism. For instance, the endoplasmic reticulum is not only the primary site for protein folding and quality control but also for lipid synthesis and storage. The study by Carrasquillo Rodríguez et al. (Molecular Biology of the Cell, 2024) elucidates how CTD-nuclear envelope phosphatase 1 (CTDNEP1) and its regulatory subunit NEP1R1 coordinate ER membrane biogenesis and lipid droplet formation through the regulation of lipin 1. This regulatory axis ensures lipid and membrane homeostasis, which is tightly coupled to protein quality control mechanisms governed by p97. The intersection of these pathways suggests that pharmacological disruption of p97 by CB-5083 not only impacts protein degradation but may also indirectly influence ER membrane dynamics and lipid metabolism—avenues that warrant further exploration in the context of cancer cell survival and stress adaptation.

Practical Guidance for Research Application

For experimental use, CB-5083 is supplied as a solid and should be stored at -20°C to maintain stability. Due to its hydrophobic nature, researchers are advised to prepare stock solutions in DMSO or ethanol, utilizing warming and ultrasonic agitation to improve solubility. CB-5083 is intended exclusively for research purposes and is not approved for diagnostic or clinical use. Researchers investigating protein homeostasis disruption, the unfolded protein response, or the caspase signaling pathway in models of multiple myeloma or solid tumors may find CB-5083 particularly valuable for elucidating p97-dependent mechanisms. Its proven efficacy in inducing cancer cell apoptosis and tumor growth inhibition in xenograft models positions it as a rigorous tool compound for dissecting the consequences of proteostasis perturbation.

Future Directions: Integration with Lipid-Protein Quality Control Networks

Given the emerging evidence linking ER lipid synthesis, storage, and protein quality control, future studies should consider the dual impact of p97 inhibition on both proteostasis and lipid metabolic pathways. The findings by Carrasquillo Rodríguez et al. (2024) regarding the NEP1R1-CTDNEP1 complex underscore the dynamic regulation of the ER environment under various metabolic conditions. Investigating whether CB-5083-mediated inhibition of p97 alters lipid homeostasis or ER membrane architecture could provide new insights into cancer cell vulnerabilities arising from the intersection of these critical cellular processes.

Conclusion

CB-5083 represents a potent, selective p97 AAA-ATPase inhibitor with demonstrated efficacy in disrupting protein homeostasis, activating the unfolded protein response, and inducing apoptosis in cancer cells. Its ability to inhibit tumor growth in xenograft models—combined with its oral bioavailability—underscores its utility for multiple myeloma research and solid tumor research. Distinct from studies like that of Carrasquillo Rodríguez et al. (2024), which focus on the regulatory networks of ER lipid synthesis and storage, this article centers on the pharmacological targeting of the protein degradation pathway via p97 inhibition. By connecting mechanistic insights with practical guidance, this review extends the discussion from ER lipid regulation to the functional consequences of protein homeostasis disruption in cancer biology—providing a complementary perspective for researchers investigating the multifaceted roles of the endoplasmic reticulum.