PARP1/2 inhibitors are a class of anticancer agents that target tumor-specific defects in DNA repair. BMN 673 is a highly potent PARP1/2 inhibitor. In vitro: BMN 673 is a potent PARP1/2 inhibitor, but it does not inhibit other enzymes that we have tested. BMN673 exhibits selective antitumor cytotoxicity and elicits DNA repair biomarkers at much lower concentrations than earlier generation PARP1/2 inhibitors. BMN 673 targeted tumor cells with BRCA1, BRCA2, or PTEN gene defects selectively with 20- to more than 200-fold greater potency than existing PARP1/2 inhibitors in vitro . In vivo: BMN 673 is readily orally bioavailable, with more than 40% absolute oral bioavailability in rats when dosed in CMC. Orally BMN 673 elicited remarkable antitumor activity in mice; xenografted tumors that carry defects in DNA repair due to BRCA mutations or PTEN deficiency were profoundly sensitive to oral BMN 673 treatment at well-tolerated doses. Synergistic antitumor effects were also found when BMN 673 was combined with temozolomide, SN38, or platinum drugs . Clinical trial: Pharmacokinetics (PK), pharmacodynamics (PD), safety and anti-tumor activity of BMN 673 were evaluated in a 2-stage dose-escalation study with 3-6 patients (pts)/dose level. Results showed BMN 673 was well tolerated with impressive anti-tumor activity in pts with BRCA mut with a single agent recommended Phase II trial dose of 1000 μg/d due to dose-limiting thrombocytopenia. Reference:  Shen Y, Rehman FL, Feng Y, Boshuizen J, Bajrami I, Elliott R, Wang B, Lord CJ, Post LE, Ashworth A. BMN 673, a novel and highly potent PARP1/2 inhibitor for the treatment of human cancers with DNA repair deficiency. Clin Cancer Res. 2013;19(18):5003-15.