related to potency and pharmacokinetics to allow for the design of an optimized dosing schedule. An optimized dosing schedule is intended to improve peak drug exposure leading to apoptosis and cell cycle arrest during the dosing period, while permitting hematopoietic precursor cell recovery during the dosing break, thereby minimizing hematologic toxicity.
Overview of p53 and MDM2
Milademetan reactivates p53, known as the “guardian of the genome,” by inhibiting MDM2. p53 is present in every normal cell and acts as a key regulator of a variety of cellular processes including cell cycle, DNA repair and apoptosis. In a normal cell, the activity of p53 is controlled and regulated by the inhibitory protein MDM2. MDM2 binds to p53, thereby inducing degradation and allowing normal cells to function properly. In response to cell damage and other stress conditions, p53 is activated and prevents the formation of cancerous cells by inducing apoptosis.
In contrast to normal cells, in tumor cells, the two primary mechanisms by which p53 can be inactivated in tumor cells are mutations in p53 and activation or overexpression of MDM2. Approximately half of all tumors are characterized by mutations of the p53 gene. The remaining cancer patients have a p53 gene that is not mutated, and is otherwise known as WT p53, but can be functionally suppressed through the activation or overexpression of MDM2. We have identified potential MDM2 dependence in several solid tumors. This dependence is caused by overexpression of MDM2 through gene amplification or other mechanisms, loss of a negative regulator of MDM2 or other causes. Overexpression of MDM2 promotes the degradation of p53 and also eliminates p53’s ability to activate transcription. Milademetan, by binding MDM2 at the p53 interaction site, prevents the formation of the MDM2-p53 complex, allowing p53 reactivation and subsequent transcription of genes that trigger cancer cell cycle arrest (e.g., p21) or apoptosis (e.g., PUMA) and can be readily measured by increases in the pharmacodynamic marker MIC-1 (GDF15) in the blood.
Clinical Development Plan
Phase 3 Trial in DDLPS Patients (MANTRA)
In July 2021, we initiated a randomized, multicenter, open-label, Phase 3 registrational trial (MANTRA) for patients with unresectable or metastatic DDLPS that has progressed on one or more prior systemic therapies, including at least one anthracycline-based therapy. This trial is an open label, 1:1 randomized trial comparing milademetan to trabectedin, a standard of care (SOC) therapeutic, planned for approximately 160 DDLPS patients with prior anthracycline-based therapy. The dosing schedule for the Phase 3 trial is identical to the Schedule D dose of 260 mg (qd 3/14x2) employed in the Phase 1 trial. The primary objective of the trial is to compare progression-free survival (PFS) evaluated by Response Evaluation Criteria in Solid Tumors (RECIST) criteria in blinded independent review between the milademetan treatment arm and the trabectedin control arm. Secondary endpoints include overall survival (OS), PFS by investigator assessment, objective response rate, duration of response, disease control rate, safety and patient reported outcomes.
The PFS assumptions for statistical powering are based on a PFS assumption for the control arm of 3.0 months and 6.0 months for milademetan. The doubling of PFS corresponds to a hazard ratio of 0.5.
In August 2022, we announced the completion of enrollment into our MANTRA Phase 3 trial of milademetan. In May 2023, we announced topline pivotal data from our MANTRA Phase 3 trial. As discussed above, the trial, evaluating the efficacy, safety, and tolerability of milademetan in patients with DD LPS, did not meet its primary endpoint of PFS by blinded independent central review compared to the standard of care, trabectedin.
Phase 2 MDM2-amplified Tumor-agnostic Basket Trial (MANTRA-2)
In November 2021, the first patient was dosed in the multicenter, single arm, open-label, Phase 2 basket trial evaluating Milademetan for the treatment of MDM2-amplified advanced solid (MANTRA-2). The MANTRA-2 trial is designed to evaluate the safety and efficacy of milademetan in patients with advanced or metastatic solid tumors refractory or intolerant to standard-of-care therapy and that exhibit WT p53 and a prespecified minimum MDM2 gene copy number. Approximately 65 patients are anticipated to be enrolled to receive milademetan. The primary endpoint of the trial is objective response rate (ORR) as measured by RECIST criteria. Secondary endpoints include duration of response, disease control rate PFS by investigator assessment, OS, and growth modulation index.
We enrolled patients that have received and progressed on SOC therapy and will be unlikely to tolerate or derive clinically meaningful benefit from SOC therapy. Patients with MDM2 amplification will be selected using local tumor testing,