Antibody-Drug Conjugate Sacituzumab Govitecan Enables a Sequential TOP1/PARP Inhibitor Therapy Strategy in Patients with Breast Cancer

Abstract

Purpose: The antibody-drug conjugate (ADC) sacituzumab govitecan (SG) comprises the topoisomerase 1 (TOP1) inhibitor (TOP1i) SN-38, coupled to a monoclonal antibody targeting trophoblast cell surface antigen 2 (TROP-2). Poly(ADP-ribose) polymerase (PARP) inhibition may synergize with TOP1i and SG, but previous studies combining systemic PARP and TOP1 inhibitors failed due to dose-limiting myelosuppression. Here, we assess the proof-of-mechanism and clinical feasibility for SG and talazoparib (TZP) employing an innovative sequential dosing schedule.

Patients and methods: In vitro models tested pharmacodynamic endpoints, and in a phase 1b clinical trial (NCT04039230), 30 patients with metastatic triple-negative breast cancer (mTNBC) received SG and TZP in a concurrent (N = 7) or sequential (N = 23) schedule. Outcome measures included safety, tolerability, preliminary efficacy, and establishment of a recommended phase 2 dose.

Results: We hypothesized that tumor-selective delivery of TOP1i via SG would reduce nontumor toxicity and create a temporal window, enabling sequential dosing of SG and PARP inhibition. In vitro, sequential SG followed by TZP delayed TOP1 cleavage complex clearance, increased DNA damage, and promoted apoptosis. In the clinical trial, sequential SG/TZP successfully met primary objectives and demonstrated median progression-free survival (PFS) of 7.6 months without dose-limiting toxicities (DLT), while concurrent dosing yielded 2.3 months PFS and multiple DLTs including severe myelosuppression.

Conclusions: While SG dosed concurrently with TZP is not tolerated clinically due to an insufficient therapeutic window, sequential dosing of SG followed by TZP proved a viable strategy. These findings support further clinical development of the combination and suggest that ADC-based therapy may facilitate novel, mechanism-based dosing strategies.

Figure 1.

Sequential dosing of SG and PARPi stabilizes TOP1CC and induces DNA damage and cell death. A, Dose-response curves of human MDA-MB-468 TNBC cells treated with different concentrations of SG for 4 hours followed by washout, then treated with either vehicle or indicated doses of TZP for 24 hours followed by medium change and assessment of viability at 72 hours. Values plotted represent the mean of quadruplicate wells and an average of three independent experiments. Error bars indicate SD. P value calculated by two-way ANOVA. *, P < 0.05. B, Sequential SG then TZP results in persistent TOP1 covalently bound to DNA, as assessed by the RADAR assay (see Patients and Methods). MDA-MB-468 cells were treated with SG (50 nmol/L) for 4 hours followed by washout, then either cultured with or without TZP (100 nmol/L) for the indicated times and processed for RADAR. Detection of dsDNA by dot-blot serves as a loading control. C, Confocal immunofluorescence with TOP1CC-specific antibody. MDA-MB-468 cells were treated with 50 nmol/L of SG for 4 hours followed by washout, then cultured with or without 100 nmol/L of TZP for the indicated times. At right, quantification of TOP1CC foci per cell. At least 50 cells were counted for each condition. Error bars indicate SD. P value calculated by two-way ANOVA. **, P < 0.01; ****, P < 0.0001; ns, no significance. D, Accumulation of DNA damage induced by sequential dosing of SG and TZP. MDA-MB-468 cells were treated as indicated, fixed and stained with anti-γ-H2AX antibody and DAPI, then analyzed by flow cytometry. Left: representative flow cytometry plots. Right: quantification of percentage of γ-H2AX positive cells from duplicate samples in two independent experiments. Error bars indicate SD. P value calculated by Student t test. *, P < 0.05; **, P < 0.01. E, Increased apoptosis induced by sequential dosing of SG and TZP. MDA-MB-468 cells were stained with propidium iodide and Annexin V following the indicated treatment. Left: representative flow cytometry plot. Right: quantification of percentage of apoptotic cells from duplicate samples in two independent experiments. Error bars indicate SD. P value calculated by Student t test. *, P < 0.05.

Figure 2.

Sequential dosing of SG and PARPi is associated with lower incidence of adverse effects and higher clinical efficacy than concurrent dosing. A, Percentage of patients experiencing DLTs and significant grade 3/4 adverse events with concurrent (blue) and sequential (red) schedule. B, Waterfall plot indicating the best percent change from baseline in the sum of the diameters of the target lesions (longest diameter for non-nodal lesions and short axis for nodal lesions), per RECIST V1.1, with sequential (left) and concurrent (right) schedule. Below are shown tumor biomarker assessments including TROP2 staining by IHC; TILs, Ki67 staining by IHC; and mutation of TP53, PTEN, PIK3CA, BRCA1/2, and CCNE1 assessed by validated plasma-based genotyping assay (Guardant³⁶⁰). C, Kaplan–Meier analysis of PFS with sequential (red line) versus concurrent (blue line) schedules. D, Swimmers’ plot illustrating time-on-treatment for patients enrolled in concurrent and sequential cohorts. Two patients remain on-treatment as of the data cutoff date. E, Distribution of TROP2 tumor cell membrane staining as quantitated by H-score (see Patients and Methods) in patients experiencing PD (n = 6) versus PR/CR (n = 17). H-score cutoffs: low (<100); medium (100–200); high (201–300). PD, progressive disease; PR, partial response; SD, stable disease.