This is an open-label, multi-center, phase 1a/b study in participants with FGFR3-altered advanced solid tumors, including metastatic urothelial cancer (UC). The study will be conducted in 2 phases: Dose escalation and dose optimization (1a) and dose expansion (1b). Phase 1a will include up to 2 cohorts to assess safety, tolerability, and pharmacokinetics of LOXO-435 to determine the recommended phase 2 dose (RP2D) (or optimal dose). Phase 1b will include 4 dose expansion cohorts of participants with prespecified activating FGFR3 alterations to evaluate the efficacy and safety of LOXO-435 at the RP2D. Cohort B will enroll pts with metastatic UC and includes three cohorts to evaluate LOXO-435 as monotherapy (B1, B2) and in combination with pembrolizumab (B3). Cohort C will enroll pts with non-UC advanced solid tumors and includes a cohort to evaluate LOXO-435 as monotherapy (C1).
Inclusion Criteria:
* Have solid tumor cancer with an FGFR3 pathway alteration on molecular testing in tumor or blood sample that is deemed as actionable.
* Cohort A1 (Dose Escalation): Presence of an alteration in FGFR3 or its ligands.
* Cohort A2 (Dose Optimization): Histological diagnosis of urothelial cancer (UC) that is locally advanced or metastatic with a qualifying FGFR3 alteration.
* Cohorts B1, B2 and B3 (Dose Expansion): Histological diagnosis of urothelial cancer that is locally advanced or metastatic with a prespecified activating FGFR3 alteration.
* Cohort C (Dose Expansion): Must have histological diagnosis of a non-urothelial solid tumor malignancy that is locally advanced or metastatic with a prespecified activating FGFR3 alteration.
* Measurability of disease:
* Cohort A1: Measurable or non-measurable disease as defined by Response Evaluation Criteria in Solid Tumors v 1.1 (RECIST v1.1)
* Cohorts A2, B1, B2, B3, and C1: Measurable disease required as defined by RECIST v1.1
* Have adequate archival tumor tissue sample available or undergo a screening biopsy if allowed per country-specific regulations.
* Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
* Prior Systemic Therapy Criteria:
* Cohort A1/C1: Participant has received all standard therapies for which the participant was deemed to be an appropriate candidate by the treating Investigator; OR the participant is refusing the remaining most appropriate standard of care treatment; OR there is no standard therapy available for the disease. There is no restriction on number of prior therapies.
* Cohort A2/B1/B2/B3: Participants must have received at least one prior regimen in the advanced or metastatic setting. There is no restriction on number of prior therapies.
* FGFR inhibitor specific requirements:
* Cohort A1/A2: Prior FGFR inhibitor treatment is permitted, but not required.
* Cohort B1: Participants must have been previously treated with a FGFR inhibitor.
* Cohort B2, B3, C1: Participants must be FGFR inhibitor naïve.
Exclusion Criteria:
* Participants with primary central nervous system (CNS) malignancy.
* Known or suspected history of uncontrolled CNS metastases.
* Current evidence of corneal keratopathy or retinal disorder.
* Have a history and/or current evidence of extensive tissue calcification.
* Any serious unresolved toxicities from prior therapy.
* Significant cardiovascular disease.
* Prolongation of the QT interval corrected for heart rate using Fridericia's formula (QTcF).
* Active uncontrolled systemic infection or other clinically significant medical conditions.
* Participants who are pregnant, lactating, or plan to breastfeed during the study or within 6 months of the last dose of study treatment. Participants who have stopped breastfeeding may be enrolled.
