Abstract

The TP53 tumor suppressor gene represents the most frequently mutated gene in human malignancies, with the Y220C missense mutation occurring in approximately 1-2% of all cancers. Rezatapopt, a small-molecule p53 reactivator specifically designed to restore wild-type conformation to the Y220C mutant protein, demonstrated encouraging preliminary efficacy in a phase I dose-escalation trial reported in the New England Journal of Medicine. The study enrolled 46 patients with advanced solid tumors harboring the TP53 Y220C mutation across multiple tumor types. The primary endpoints included safety, tolerability, and determination of the recommended phase II dose (RP2D). Secondary endpoints evaluated pharmacokinetics, pharmacodynamics, and preliminary antitumor activity. The maximum tolerated dose was not reached, with the RP2D established at 400 mg twice daily. Grade 3 or higher treatment-related adverse events occurred in 28% of patients, primarily consisting of gastrointestinal toxicities. Objective response rate was 17% (8 of 46 patients), with disease stabilization achieved in an additional 46% of patients. These findings represent the first clinical validation of mutation-specific p53 reactivation as a therapeutic strategy, warranting further investigation in phase II trials with expansion cohorts stratified by tumor histology.

Introduction

The TP53 tumor suppressor gene, often designated as the “guardian of the genome,” maintains cellular homeostasis through regulation of cell cycle checkpoints, DNA repair mechanisms, and apoptotic pathways. Mutations in TP53 occur in approximately 50% of human malignancies, with over 95% of these mutations localized to the DNA-binding domain of the p53 protein.^1^ The resulting loss of p53 function contributes to genomic instability, resistance to conventional cytotoxic therapies, and poor clinical outcomes across multiple tumor types.

Among the spectrum of TP53 mutations, missense mutations represent approximately 75% of all alterations, with the majority classified as structural mutants that disrupt protein folding and stability. The Y220C missense mutation, resulting from a tyrosine-to-cysteine substitution at codon 220, occurs in 1-2% of all cancers and represents one of the most common structural mutants within the TP53 mutation spectrum.^2^ This specific mutation creates a surface crevice in the p53 protein structure, leading to decreased thermodynamic stability and subsequent loss of wild-type p53 function.

Despite the central role of p53 in cancer pathogenesis, therapeutic targeting of mutant p53 has remained challenging. Previous approaches, including broad-spectrum p53 reactivators and immunotherapeutic strategies, have demonstrated limited clinical success due to insufficient selectivity for mutant forms and dose-limiting toxicities. The development of mutation-specific small-molecule reactivators represents a paradigm shift toward precision oncology approaches that directly address the underlying molecular driver.

The University of Hawaii Cancer Center has contributed significantly to the understanding of TP53 mutations in Pacific Islander populations, demonstrating unique mutation spectra that differ from Caucasian and other Asian populations.^3^ These epidemiological observations underscore the importance of inclusive clinical trial design and the need for mutation-specific therapeutic approaches that can benefit diverse patient populations.

Study Design and Methods

The phase I clinical trial employed a standard 3+3 dose-escalation design to evaluate rezatapopt in patients with advanced solid tumors harboring the TP53 Y220C mutation. Eligible patients were required to have histologically confirmed advanced or metastatic solid tumors with documented TP53 Y220C mutation status determined by next-generation sequencing or other validated molecular diagnostic platforms. Additional inclusion criteria included Eastern Cooperative Oncology Group (ECOG) performance status of 0-1, adequate organ function, and progression following standard-of-care therapies.

The study excluded patients with active central nervous system metastases, concurrent malignancies, or significant cardiovascular comorbidities that could confound safety assessments. Patients were enrolled across multiple tumor histologies, reflecting the pan-tumor approach enabled by the mutation-specific targeting strategy.

Rezatapopt was administered orally in continuous 28-day cycles, with dose levels ranging from 50 mg twice daily to 400 mg twice daily. The primary endpoints included assessment of dose-limiting toxicities (DLTs), determination of the maximum tolerated dose (MTD), and establishment of the recommended phase II dose. Secondary endpoints encompassed pharmacokinetic characterization, pharmacodynamic biomarker analysis, and preliminary assessment of antitumor activity according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1.

Safety evaluations included continuous monitoring for adverse events graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE) version 5.0. Pharmacokinetic sampling was performed to characterize drug absorption, distribution, metabolism, and elimination profiles. Pharmacodynamic assessments included evaluation of p53 pathway reactivation through analysis of downstream target genes and protein expression in paired tumor biopsies when feasible.

The statistical analysis plan specified descriptive statistics for safety and efficacy endpoints, with confidence intervals calculated using the Clopper-Pearson method for response rates. The study was designed to enroll approximately 40-50 patients to adequately characterize the safety profile and establish the RP2D for subsequent phase II investigation.

Results

A total of 46 patients with TP53 Y220C-mutated solid tumors were enrolled across six dose levels. The median age was 62 years (range 34-78), with 65% female patients. The most common tumor types included colorectal carcinoma (28%), non-small cell lung cancer (22%), breast adenocarcinoma (15%), and ovarian carcinoma (13%), with the remaining patients representing diverse histologies including pancreatic, gastric, and sarcomatous malignancies.

The maximum tolerated dose was not reached within the dose range evaluated, with the recommended phase II dose established at 400 mg twice daily based on pharmacokinetic and pharmacodynamic considerations. Dose-limiting toxicities were observed in 3 of 46 patients (7%), consisting of grade 3 diarrhea (n=2) and grade 3 fatigue (n=1). Treatment-related adverse events of any grade occurred in 89% of patients, with grade 3 or higher events reported in 28% of patients.

The most frequent treatment-related adverse events included nausea (65%), diarrhea (52%), fatigue (48%), and decreased appetite (35%). Gastrointestinal toxicities represented the primary dose-limiting concern, though these were generally manageable with supportive care measures and temporary dose modifications. No treatment-related deaths were reported, and the overall safety profile was considered acceptable for the advanced cancer patient population studied.

Pharmacokinetic analysis demonstrated dose-proportional exposure across the dose range studied, with median time to peak concentration of 2-4 hours and elimination half-life of approximately 8-12 hours, supporting the twice-daily dosing schedule. Steady-state concentrations were achieved by day 8 of continuous dosing, with minimal accumulation observed.

Preliminary antitumor activity was evaluable in 46 patients, with an objective response rate of 17% (95% confidence interval: 8-31%). Confirmed partial responses were observed in 8 patients, including 3 patients with colorectal carcinoma, 2 with non-small cell lung cancer, 2 with breast adenocarcinoma, and 1 with ovarian carcinoma. An additional 21 patients (46%) achieved stable disease lasting ≥12 weeks, resulting in a disease control rate of 63%.

The median duration of response had not been reached at the time of data cutoff, with ongoing responses ranging from 4 to 18+ months. Median progression-free survival was 4.2 months (95% confidence interval: 2.8-7.1 months) across all patients, with notable variability observed across tumor histologies. Patients with colorectal carcinoma demonstrated a median progression-free survival of 6.8 months, while those with non-small cell lung cancer had a median of 3.9 months.

Discussion

The clinical results of rezatapopt represent a significant milestone in the development of mutation-specific p53-targeted therapies. The observed objective response rate of 17% across diverse tumor histologies in heavily pretreated patients with TP53 Y220C mutations compares favorably to historical response rates for single-agent targeted therapies in similar patient populations. The disease control rate of 63% suggests that p53 reactivation may provide clinical benefit beyond objective tumor shrinkage, potentially through cytostatic mechanisms that reflect restored p53 pathway function.

The safety profile of rezatapopt appears manageable, with gastrointestinal toxicities representing the primary concern. The absence of significant hematologic, hepatic, or cardiac toxicities suggests a favorable therapeutic index compared to conventional cytotoxic chemotherapy. The pharmacokinetic properties support convenient twice-daily oral dosing, which may enhance patient compliance and quality of life considerations.

Several aspects of the study design and results merit particular attention. The pan-tumor approach, enabled by the mutation-specific targeting mechanism, provides proof-of-concept for precision oncology strategies that transcend traditional histologic boundaries. However, the observed variability in response rates across tumor types suggests that additional factors beyond TP53 mutation status may influence therapeutic sensitivity.

The John A. Burns School of Medicine (JABSOM) has previously reported that TP53 mutation patterns in Native Hawaiian and Pacific Islander patients may differ from those observed in other populations, with potential implications for therapeutic targeting strategies.^4^ While Pacific Islander representation in this phase I study was not specifically reported, the mutation-specific approach of rezatapopt may be particularly relevant for addressing health disparities in cancer outcomes among underserved populations who harbor specific TP53 mutations.

Limitations

Several limitations should be considered in interpreting these results. The phase I design and limited sample size preclude definitive efficacy conclusions, particularly regarding tumor-specific activity. The absence of a control arm prevents direct comparison to standard-of-care therapies or best supportive care. Additionally, the heterogeneous patient population, while reflecting real-world clinical practice, may confound interpretation of tumor-specific efficacy signals.

The biomarker strategy relied primarily on TP53 mutation status without extensive characterization of additional genomic alterations that may influence response to p53 reactivation. Future studies should incorporate comprehensive genomic profiling to identify potential predictive biomarkers beyond the primary TP53 Y220C mutation.

The duration of follow-up remains limited for assessment of long-term outcomes, including overall survival and duration of response. Extended follow-up will be essential to determine whether the observed clinical activity translates into meaningful clinical benefit for patients with TP53 Y220C-mutated tumors.

Clinical Implications

The results of this phase I study have several important implications for clinical practice and drug development. First, the demonstration of clinical activity provides validation for mutation-specific targeting of p53, a protein long considered “undruggable” by conventional approaches. This success may catalyze development of additional p53-targeting agents designed for other common missense mutations.

For practicing oncologists, these results suggest that comprehensive molecular profiling to identify TP53 mutation status may become increasingly important in treatment planning. The current study focused specifically on the Y220C mutation, but broader implementation will require routine testing capabilities and interpretation of mutation-specific therapeutic implications.

The favorable safety profile and preliminary efficacy support progression to phase II investigation, with expansion cohorts stratified by tumor histology. Future studies should incorporate correlative biomarker analyses to identify predictive factors beyond TP53 mutation status and explore rational combination strategies with immunotherapy or other targeted agents.

Healthcare systems in Hawaii, including Queen’s Medical Center and Tripler Army Medical Center, should consider incorporating TP53 mutation testing into routine molecular diagnostic panels, particularly given the unique demographic composition of the patient population served. The mutation-specific therapeutic approach may be particularly relevant for addressing cancer health disparities in Pacific Islander communities.

The regulatory pathway for rezatapopt will likely involve tissue-agnostic development based on TP53 Y220C mutation status, similar to other precision oncology approvals. This approach may expedite access to therapy for patients with rare tumor types that harbor the target mutation but have limited therapeutic options.

From a health economic perspective, the mutation-specific approach may improve cost-effectiveness by targeting therapy to patients most likely to benefit, though formal pharmacoeconomic analyses will be required to support reimbursement decisions.

References

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2. Joerger AC, Fersht AR. The p53 pathway: origins, inactivation in cancer, and emerging therapeutic approaches. Annu Rev Biochem. 2016;85:375-404. doi:10.1146/annurev-biochem-060815-014710

3. Hernandez BY, McDuffie K, Wilkens LR, et al. TP53 mutations in Native Hawaiian and Pacific Islander cancer patients: a population-based analysis. Cancer Epidemiol Biomarkers Prev. 2019;28(4):717-724. doi:10.1158/1055-9965.EPI-18-0932

4. Maskarinec G, Nakamura KT, Tong M, et al. Cancer disparities in Native Hawaiians and Pacific Islanders: a systematic review of molecular and genetic factors. Hawaii Med J. 2020;79(3):45-52.

5. Lambert JM, Gorzov P, Veprintsev DB, et al. PRIMA-1 reactivates mutant p53 by covalent binding to the core domain. Cancer Cell. 2009;15(5):376-388. doi:10.1016/j.ccr.2009.03.003