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Background: \ud We have previously shown that raised p-S6K levels correlate with resistance to chemotherapy in ovarian cancer. We hypothesised that inhibiting p-S6K signalling with the dual m-TORC1/2 inhibitor in patients receiving weekly paclitaxel could improve outcomes in such patients.\ud \ud Patients and methods: \ud In dose escalation, weekly paclitaxel (80 mg/m2) was given 6/7 weeks in combination with two intermittent schedules of vistusertib (dosing starting on the day of paclitaxel): schedule A, vistusertib dosed bd for 3 consecutive days per week (3/7 days) and schedule B, vistusertib dosed bd for 2 consecutive days per week (2/7 days). After establishing a recommended phase II dose (RP2D), expansion cohorts in high-grade serous ovarian cancer (HGSOC) and squamous non-small-cell lung cancer (sqNSCLC) were explored in 25 and 40 patients, respectively.\ud \ud Results: \ud The dose-escalation arms comprised 22 patients with advanced solid tumours. The dose-limiting toxicities were fatigue and mucositis in schedule A and rash in schedule B. On the basis of toxicity and pharmacokinetic (PK) and pharmacodynamic (PD) evaluations, the RP2D was established as 80 mg/m2 paclitaxel with 50 mg vistusertib bd 3/7 days for 6/7 weeks. In the HGSOC expansion, RECIST and GCIG CA125 response rates were 13/25 (52%) and 16/25 (64%), respectively, with median progression-free survival (mPFS) of 5.8 months (95% CI: 3.28–18.54). The RP2D was not well tolerated in the SqNSCLC expansion, but toxicities were manageable after the daily vistusertib dose was reduced to 25 mg bd for the following 23 patients. The RECIST response rate in this group was 8/23 (35%), and the mPFS was 5.8 months (95% CI: 2.76–21.25).\ud \ud Discussion: \ud In this phase I trial, we report a highly active and well-tolerated combination of vistusertib, administered as an intermittent schedule with weekly paclitaxel, in patients with HGSOC and SqNSCLC.\ud \ud Clinical trial registration: \ud ClinicialTrials.gov identifier: CNCT02193633.

2573 Background: The phosphatidylinositol 3-kinase (PI3K) pathway is commonly mutated in cancer. Tas is a selective β-isoform-sparing PI3K inhibitor with improved therapeutic index compared to pan-PI3K inhibitors. Palb is a CDK4/6 inhibitor now standard of care in combination with endocrine therapy (ET) in hormone receptor positive breast cancer. Combination of Tas, Palb and ET is synergistic in preclinical models. Methods: This investigator-initiated study investigated safety and tolerability, pharmacokinetics (PK), PD and antitumor activity of Tas+Palb, with addition of ET in dose expansion. Pts were enrolled in 3+3 dose escalation design. Tas was given continuously or 3-weeks-on, 1-week-off (3/1), Palb was given on 3/1 schedule. PD studies included analyses of platelet-rich plasma (PRP) (n = 20) and paired tumor biopsies (n = 5). Serial circulating tumor DNA was monitored in pts with PIK3CA mutations. Results: 24 pts were treated, 22 with Tas+Palb, 2 with Tas+Palb+fulvestrant(ful); M/F 11/13, median lines prior therapy 4. Treatment was well tolerated with mainly G1-2 toxicities. Most frequent G3 toxicities were neutropenia (5/24), thrombocytopenia (5/24) and rash (5/24), with no G4/5 toxicities. Two pts had dose-limiting toxicities (DLT) at DL2. No DLTs were observed at DL4, although pts experienced delayed neutrophil recovery. PK was linear and comparable with monotherapy. At 125mg Palb, significant decreases in pAKT and pGSK3β in PRP confirmed PI3K target inhibition. Two pts with PI3KCA H1047R mutant breast cancers have ongoing RECIST partial response; 1 pt with PIK3CA E542K colorectal cancer had stable disease for 20 weeks. Conclusions: Tas+Palb is well tolerated with evidence of PD and antitumor activity. Dose expansion including recruitment to triplet Tas+Palb+ful and Tas+Palb+letrozole is ongoing with continuous Tas 2mg QD, and Palb 100mg QD on 3/1 schedule, increasing to 125mg after cycle 1 in absence of myelosuppression. Clinical trial information: NCT02389842. [Table: see text]