Background: Genetically stratified therapy for malignant mesothelioma (MM) is lacking. MMs frequently harbour loss of chromosome 9p21.3 locus (CDKN2A) associated with shorter survival. CDKN2A encodes the tumour suppressor p16ink4a, an endogenous suppressor of CDK4 and CDK6. Genetic reconstitution of p16ink4a into CDKN2A suppresses MM in preclinical models, underpinning the rationale for targeting of CDK4/6 in p16ink4a negative MM. We therefore developed a multi-centre molecularly stratified phase IIa trial to test this hypothesis as arm 2 of the Mesothelioma Stratified Therapy umbrella trial (NCT03654833, MiST2). Methods: Patients with histologically confirmed MM (pleural or peritoneal) were molecularly screened by immunohistochemistry for p16ink4a, BAP1, BRCA1, and PDL1 (Dako 22C3). Patients with p16ink4a negative MM were eligible. Key inclusion factors: histological confirmation of MM with an available archival tissue block, ECOG performance status 0-1, prior platinum-based 1^st line chemotherapy (any line allowed), evidence of disease progression with measurable disease by CT (RECIST 1.1), and adequate haematological/organ function. Patients received Abemaciclib (Ab) 200mg bd po daily in q21 day cycles. Primary endpoint was disease control rate at 12 weeks (DCR12w). The null hypothesis was rejected if ≥ 11 patients had disease control (A’Hern design). Secondary endpoints: DCR at 24 weeks (DCR24w), best objective response rate and toxicity (NCI CTCAE 4.03). Patients could undergo an optional re-biopsy upon disease progression. Results: Between November 2019 and March 2020, 26 patients with p16ink4a negative MM received at least one dose of Ab. The median age of pts was 67 (range, 64-74) years, 89% were male, 80.8% epithelioid, 84.6% ECOG PS1, > 1 prior systemic therapy 62%. All patients received at least one cycle with 27% completing 6 cycles. No dose reductions occurred in 53.9% of pts. DCR12w: 54% (95% confidence limit (CI), 36% – 71%), DCR24w: 23% (95%CI, 9% – 44%). Best responses (within 24w): partial - 15% (95%CI, 4- 35%), one occurring after 18 weeks; stable disease - 53.8% (33.4 – 73.4%); progression - 7.7% (0.9 – 25.1%). Adverse events (any cause): ≥ grade 3 toxicities affected 5.7% of pts. Conclusions: MiST2 met its primary endpoint, warranting further clinical evaluation of Ab. Whole exome sequencing of the cohort is ongoing to explore genomic correlates of de novo and acquired resistance. Clinical trial information: NCT03654833