Background: The PI3K pathway may be a potential mechanism to overcome cisplatin resistance. We conducted a phase Ib trial of alpelisib and cisplatin for patients with solid tumor malignancies with planned dose expansion in HPV-associated tumors. Methods: The primary objective was to determine the maximally tolerated dose (MTD) and recommend phase II dose. Two different weekly doses of cisplatin (30 and 35 mg/m2) were evaluated with escalating doses of alpelisib, administered daily during a 21-day treatment cycle. Results: 23 patients were enrolled: 91% received >3 prior regimens with median of 4 (range 1-10), and 78% progressed on prior platinum. The MTD was alpelisib 250 mg daily with weekly cisplatin 30 mg/m2. There were 3 DLTs: all grade 4 hyperglycemia. Frequent treatment-related adverse events of any grade included fatigue (52%), diarrhea (39%), nausea (38%), hyperglycemia (30%), anemia (22%), and nephropathy (17%). Hyperglycemia was linked to baseline hemoglobin A1C, but not body mass index. 12 patients discontinued treatment for toxicity (n=9 during cycle 1) and 11 discontinued for progression. Of 14 evaluable patients who received at least one treatment cycle, 4 (29%) patients demonstrated partial response, and 7 had stable disease for a disease control rate of 79%. The median PFS measured 4.3 months (95% CI 1.6-4.5). No difference in PFS was observed between PIK3CA mutated and wild-type tumors. Conclusion: While the combination of alpelisib and cisplatin demonstrated preliminary evidence of activity despite platinum resistance, toxicities hindered prolonged treatment. Prospective studies are planned using carboplatin and alpelisib to improve toxicity and tolerability.