A phase I study to evaluate the safety, pharmacokinetics, and pharmacodynamics of PF-06939999 (PRMT5 inhibitor) in patients with selected advanced or metastatic tumors with high incidence of splicing factor gene mutations

BACKGROUND

Protein arginine methyltransferase 5 (PRMT5) methylates multiple substrates dysregulated in cancer, including spliceosome machinery components. PF-06939999 is a selective small-molecule PRMT5 inhibitor.

PATIENTS AND METHODS

This phase I dose-escalation and -expansion trial (NCT03854227) enrolled patients with selected solid tumors. PF-06939999 was administered orally once or twice a day (q.d./b.i.d.) in 28-day cycles. The objectives were to evaluate PF-06939999 safety and tolerability to identify maximum tolerated dose (MTD) and recommended part 2 dose (RP2D), and assess pharmacokinetics (PK), pharmacodynamics [changes in plasma symmetric dimethylarginine (SDMA) levels], and antitumor activities.

RESULTS

In part 1 dose escalation, 28 patients received PF-06939999 (0.5 mg q.d. to 6 mg b.i.d.). Four of 24 (17%) patients reported dose-limiting toxicities: thrombocytopenia (n = 2, 6 mg b.i.d.), anemia (n = 1, 8 mg q.d.), and neutropenia (n = 1, 6 mg q.d.). PF-06939999 exposure increased with dose. Steady-state PK was achieved by day 15. Plasma SDMA was reduced at steady state (58%-88%). Modulation of plasma SDMA was dose dependent. No MTD was determined. In part 2 dose expansion, 26 patients received PF-06939999 6 mg q.d. (RP2D). Overall (part 1 + part 2), the most common grade ≥3 treatment-related adverse events included anemia (28%), thrombocytopenia/platelet count decreased (22%), fatigue (6%), and neutropenia (4%). Three patients (6.8%) had confirmed partial response (head and neck squamous cell carcinoma, n = 1; non-small-cell lung cancer, n = 2), and 19 (43.2%) had stable disease. No predictive biomarkers were identified.

CONCLUSIONS

PF-06939999 demonstrated a tolerable safety profile and objective clinical responses in a subset of patients, suggesting that PRMT5 is an interesting cancer target with clinical validation. However, no predictive biomarker was identified. The role of PRMT5 in cancer biology is complex and requires further preclinical, mechanistic investigation to identify predictive biomarkers for patient selection.

Statistical justification of expansion cohorts in phase 1 cancer trials

BACKGROUND

Phase I cancer trials increasingly incorporate dose-expansion cohorts (DECs), reflecting a growing demand to acquire more information about investigational drugs. Protocols commonly fail to provide a sample-size justification or analysis plan for the DEC. In this study, we develop a statistical framework for the design of DECs.

METHODS

We assume the maximum tolerated dose (MTD) for the investigational drug has been identified in the dose-escalation stage of the trial. We use the 80% lower confidence bound and the 90% upper confidence bound for the response and toxicity rates, respectively, as decision thresholds for the dose-expansion stage. We calculate the operating characteristics with reference to prespecified minimum effective response rates and maximum safe DLT rates.

RESULTS

We apply our framework to specify a system of DEC plans. The design comprises three components: 1) the number of subjects enrolled at the MTD, 2) the minimum number of responses necessary to indicate provisional drug efficacy, and 3) the maximum number of dose-limiting toxicities (DLTs) permitted to indicate drug safety. We demonstrate our method in an application to a cancer immunotherapy trial.

CONCLUSIONS

Our simple and practical tool enables creation of DEC designs that appropriately address the safety and efficacy objectives of the trial.

Dose expansion cohorts in Phase I trials

A rapidly increasing number of Phase I dose-finding studies, and in particular those based on the standard 3+3 design, are being prolonged with the inclusion of dose expansion cohorts (DEC) in order to better characterize the toxicity profiles of experimental agents and to study disease-specific cohorts. These trials consist of two phases: the usual dose escalation phase that aims to establish the maximum tolerated dose (MTD), and the dose expansion phase that accrues additional patients, often with different eligibility criteria, and where additional information is collected. Current protocols do not always specify whether and how the MTD will be updated in light of the new data accumulated from the DEC. In this paper, we propose methods that allow monitoring of safety in the DEC by re-evaluating the MTD in light of additional information. Our working assumption is that, regardless of the design being used for dose escalation, during the DEC we are experimenting in the neighborhood of a target dose with an acceptable rate of toxicity. We refine our initial estimate of the MTD by continuing experimentation in the immediate vicinity of the initial estimate of the MTD. The auxiliary information provided in such an evaluation can include toxicity, pharmacokinetic, efficacy or other endpoints. We consider approaches specifically focused on the aims of DEC that examine efficacy alone or simultaneously with safety and compare the proposed tests via simulations.

Sequential monitoring of Phase I dose expansion cohorts

A relatively recent development in the design of Phase I dose-finding studies is the inclusion of expansion cohort(s), that is, the inclusion of several more patients at a level considered to be the maximum tolerated dose established at the conclusion of the 'pure' Phase I part. Little attention has been given to the additional statistical analysis, including design considerations, that we might wish to consider for this more involved design. For instance, how can we best make use of new information that may confirm or may tend to contradict the estimate of the maximum tolerated dose based on the dose escalation phase. Those patients included during the dose expansion phase may possess different eligibility criteria. During the expansion phase, we will also wish to have an eye on any evidence of efficacy, an aspect that clearly distinguishes such studies from the classical Phase I study. Here, we present a methodology that enables us to continue the monitoring of safety in the dose expansion cohort while simultaneously trying to assess efficacy and, in particular, which disease types may be the most promising to take forward for further study. The most elementary problem is where we only wish to take account of further toxicity information obtained during the dose expansion cohort, and where the initial design was model based or the standard 3+3. More complex set-ups also involve efficacy and the presence of subgroups. Copyright © 2016 John Wiley & Sons, Ltd.