The Inspire Study in Higher-Risk Myelodysplastic Syndrome (HR-MDS): A Novel Phase 3 Study Adaptive Design for Hematological Malignancies in Adults

Background:

Patients with HR-MDS have a dismal prognosis after failure of hypomethylating agents (HMAs) (Zeidan 2014), with median overall survival (OS) of less than 6 months (Prebet 2011) and currently no approved second-line therapy (Garcia-Manero 2016). Targeted therapies with novel mechanisms of action, combination strategies, as well as innovative study designs, are all needed to expedite and address the high unmet medical need in patients with HMA refractory HR-MDS. Rigosertib is a unique targeted therapy that inhibits PI3K and PLK signaling pathways by binding directly to the Ras-binding domain (Athuluri-Divakar 2016) and in vitro cytotoxicity studies have demonstrated synergy with azacitidine (Cosenza 2015). INSPIRE study is an example of a study with a novel compound and unique mechanism of action as well as an innovative study design.

Methods:

INSPIRE (NCT02562443) is a global randomized Phase 3 trial in patients with HR-MDS after HMA failure. Patients are randomized in a 2:1 fashion to rigosertib or physician's choice of treatment. Key inclusion criteria: age <82 years; MDS classified as RAEB-1, RAEB-2 or RAEB-t; ≥1 cytopenia; patient must demonstrate one of the following: progression any time after initiation of HMA treatment, intolerance to HMA, failure to achieve complete remission (CR), partial remission (PR), or hematologic improvement (HI) after six 4-week cycles of AZA or either four 4-week or four 6-week cycles of DAC, or relapse after initial CR, PR or HI; duration of prior HMA ≤9 cycles within 12 months; last dose of HMA ≤6 months before enrollment; and ECOG score 0-2. The primary endpoint of overall survival (OS) will be tested in a sequential fashion in the intention to treat (ITT) population and the IPSS-R very high risk (VHR) subgroup. Secondary endpoints include OS in patients with monosomy 7 or trisomy 8, overall response, quality-of-life, and HI. The initial sample size was 225 patients with a pre-planned interim analysis (IA) after 88 deaths. INSPIRE featured an adaptive trial design with a pre-planned Sample Size Re-estimation (Cui 1999) to 360 patients and 288 OS events as one option that the Independent Data Monitoring Committee (IDMC) could implement at IA. The adaptive sample size modification of an on-going two-arm, group sequential clinical trial is used to categorize the results for each population into three zones of Unfavorable, Promising, and Favorable. This adaptive design is advantageous as it allows study sample size to be adjusted when there is high variance in estimating the true effect of the drug under investigation which could otherwise result in underpowering of the study. The IDMC had several options following the interim analysis, including continuation of the study as initially planned, discontinuation for futility or safety, trial expansion using pre-planned sample size re-estimation, and continuation for only the pre-defined VHR subgroup. The investigators remain blinded to the specific interim analysis results. Enrollment in the trial is ongoing with topline data expected in 2020.

Conclusions:

Based on the results of interim analysis, the IDMC recommended continuation of the trial based on ITT result in promising zone with one-time expansion in enrollment, using a pre-planned sample size re-estimation, the sample size for the study was increased from 225 to 360 with unchanged eligibility criteria. The Adaptive Design used real time data from the IA to modify sample size and mitigate the risk of underpowering without undermining its validity and integrity, while preserving type-1 error. The study design used in INSPIRE may be advantageous for other novel agents in rare hematological diseases during the transition from phase 2 to phase 3 studies. Clinical trial information: (NCT02562443).