Background: Malignant Mesothelioma (MM) remains an incurable cancer lacking effective treatments in the relapsed setting. Personalised therapy is still in its infancy. Homologous recombination (HR) deficiency associated with BRCA1 mutation has been shown to predict sensitivity to inhibition of poly-ADP ribose. In MM, BRCA1 associated protein 1 carboxy-terminal hydrolase (BAP1) is frequently mutated. It regulates both HR, and BRCA1 expression which is lost in 38% of mesotheliomas. Mesothelioma Stratified Therapy 1 (MiST1) was designed to test the hypothesis that BAP1/BRCA1 negative mesotheliomas would exhibit defective HR and exhibit sensitivity to PARP inhibition. Methods: MiST1 was a single centre, open label single arm phase IIa design with prospective molecular stratification; cytoplasmic/negative BAP1 or BRCA1 deficient MM was deemed eligible. Treatment was 600mg BD rucaparib (R) PO daily every 28 days for 6 cycles or until disease progression, unacceptable toxicity, withdrawal or death. Primary outcome was disease control rate at 12 weeks (DCR12w); secondary outcomes safety and toxicity profile, objective response rate (ORR) and DCR at 24 weeks (DCR24w). The null hypothesis states the true DCR12w is less than or equal to 25% and was tested against a one-sided alternative hypothesis that the DCR12w will be equal to or greater than 50%. Results: Between February 2019 and June 2019, 26 patients (pts) were eligible and consented to MiST1, median age 65.5 years, 85% are male and 15% female. Of these pts 15% had an ECOG performance status (PS) of 0 and 85% had an ECOG PS of 1. Molecular eligibility was 89% for BAP1 alone, 50% BRCA1 alone, and 39% BAP1+BRCA1. Primary tumour site was thoracic (96%) and subtype epithelioid (81%). DCR12w was 57.7% (95% CI, 36.9 - 76.7), DCR24w was 23.1% (95% CI, 9.0 – 43.7) and ORR was 11.5% (95%CI, 2.5 – 30.2). R was well tolerated with 9% (15/166) grade (G) 3/4 toxicities seen in 10 pts (38%), with no G5 toxicities. The most common adverse events were nausea occurring in 18 pts (69%), fatigue in 14 pts (54%), and decreased appetite in 10 pts (38%). Six cycles of R was received by 8 pts (30.8%). Dose reductions occurred in 9 pts (n = 8; 1 dose and n = 1; 2 doses). Dose delays occurred in 14 pts. Conclusions: MiST1 using the PARP inhibitor R met its primary endpoint of disease control rate at 12 weeks, showing promising efficacy with manageable toxicity. HR deficiency mutation signature enrichment is being investigated to refine the identification of responders to PARP inhibition. PARP inhibition warrants further investigation in MM. Clinical trial information: NCT03654833