This is a first-in-man trial, in which BYL719 will be administered to adult patients with advanced solid tumors, whose tumors have an alteration of the PIK3CA gene and whose disease has progressed despite standard therapy or for whom no standard therapy exists. A combination of BYL719 with fulvestrant will also be investigated in post-menopausal patients with locally advanced or metastatic breast cancer whose tumors have an alteration of the PIK3CA gene. The single agent MTD dose expansion cohort and the fulvestrant combination MTD dose expansion cohort will also include ER+/HER2- breast cancer patients whose tumors have the wild type PIK3CA gene

Inclusion Criteria:

• Patients with histologically-confirmed, advanced unresectable solid tumors who have progressed within three months before screening/baseline visit Only patients who have confirmed PIK3CA status (wild type, mutation or amplification) will be allowed for screening (patients participating in the combination arm must be eligible for treatment with fulvestrant)
• Availability of a representative formalin fixed paraffin embedded tumor tissue sample
• At least one measurable or non-measurable lesion
• Age ≥ 18 years
• World Health Organization (WHO) Performance Status ≤ 2
• Good organ (hepatic, kidney, BM) function at screening/baseline visit

Exclusion Criteria:

• Brain metastasis unless treated and free of signs/symptoms attributable to brain metastasis in the absence of corticosteroid therapy (anti-epileptic therapy is allowed).
• Prior treatment with PI3K, AKT or mTOR inhibitor and failure to benefit
• Patient with peripheral neuropathy NCI-CTC Grade ≥ 3
• Patient with diarrhea NCI-CTC Grade ≥ 2
• Patient with acute or chronic pancreatitis
• Impaired cardiac function or clinically significant cardiac disease incl. unstable angina pectoris ≤ 3 months prior to starting study drug and Acute Myocardial Infarction (AMI) ≤ 3 months prior to starting study drug.
• Patients with clinically manifest diabetes mellitus, history of gestational diabetes mellitus or documented steroid-induced diabetes mellitus
• Women who are pregnant or breast feeding or adults of reproductive potential not employing an effective method of birth control

Other protocol-defined inclusion/exclusion criteria may apply