Changes in gastric pH due to oral co-administration of acid-reducing agents, such as proton pump inhibitors, and oncology drugs can have a marked impact on the absorption and oral bioavailability of the oncology drug if the pH decreases the maximal absorbable dose. Moreover, there is a potential for specific drug-drug interactions between some of the acid-reducing agents and these drugs. Methods are described for treating patients on gastric acid-reducing therapy with orally-administered PI3K inhibitor compound GDC-0941. Treatment with a re-acidification compound during treatment with GDC-0941 improves bioavailability of GDC-0941.