Oyewale O. Abidoye

Atezolizumab, a humanized immunoglobulin G1 (IgG1) monoclonal antibody targeting human programmed death-ligand 1 (PD-L1), is US Food and Drug Administration (FDA) approved in metastatic urothelial carcinoma (MUC) and is being investigated in various malignancies. This analysis based upon 906 patients from two phase I and one phase II MUC studies, is the first report of the clinical pharmacokinetics (PK) and pharmacodynamics (PD) of atezolizumab. Atezolizumab exhibited linear PK over a dose range of 1-20 mg/kg, including the labeled 1,200 mg dose. The clearance, volume of distribution, and terminal half-life estimates from population pharmacokinetic (PopPK) analysis of 0.200 L/day, 6.91 L, and 27 days, respectively, were as expected for an IgG1. Exposure-response analyses did not identify statistically significant relationships with either objective response rate or adverse events of grades 3-5 or of special interest. None of the statistically significant covariates from PopPK (body weight, gender, antitherapeutic antibody, albumin, and tumor burden) would require dose adjustment.

355 Background: The ORR and survival of mUC patients (pts) who progress after platinum-based chemotherapy (pctx) are poor. Atezolizumab (atezo) reinvigorates antitumor immunity by targeting PD-L1 and has shown promising Ph 1 activity in mUC. Methods: IMvigor 210 cohort 2 (NCT02108652) enrolled 316 mUC pts who progressed during or following pctx. Pts received atezo at 1200 mg IV q3w until loss of clinical benefit. The SP142 IHC assay centrally assessed PD-L1 expression. Pts/investigators were blinded to PD-L1 status. Co-primary endpoints were confirmed ORR by RECIST v1.1 per central review (IRF) and modified (m) RECIST per investigator, which were met if null hypothesis (ORR = 10%) was rejected (α = 5%). ORR endpoints were stratified by PD-L1 tumor-infiltrating immune cell (IC) status: IC2/3, IC1/2/3, all comers. Results: Efficacy/safety-evaluable pts (N = 311) had a median age of 66 y, CrCl < 60 mL/min (35%) and ≥ 2 prior regimens for mUC (40%). Many had poor prognostic factors (Table). At 5/5/15 data cutoff (follow up ≥ 24 w), 43/47 responding pts had ongoing responses. Both IRF (Table) and mRECIST ORR correlated with IC status. Durable responses were seen including poor prognostic subgroups (Table). mDOR was not reached in any PD-L1 or prognostic subgroup. mPFS was 2.1 mo in all PD-L1 subgroups. Median treatment duration was 12 w (range 0-46). Treatment-related AEs (most commonly fatigue) occurred in 66% of pts (all Grade); 15% had G3-4 related AE and 4% had G3-4 immune-mediated AE. 27% of AEs led to dose interruption and 3% led to withdrawal. No renal toxicity was seen. Conclusions: IMvigor 210, the first Ph 2 study targeting PD-L1/PD-1 in mUC, demonstrated significantly improved ORR vs historic controls. Responses were durable and associated with higher PD-L1 IC expression; poor prognostic factors did not preclude response. Atezo was well tolerated, and a randomized Ph 3 study vs ctx is ongoing (IMvigor 211; NCT02302807). Clinical trial information: NCT02108652. [Table: see text]