Positive outcomes in cancer therapy require drugs to reach sufficient intracellular concentrations to engage their targets. In high-grade serous ovarian carcinoma (HGSOC), PARP inhibitors have significantly improved patient response, yet resistance remains a major challenge for this highly heterogeneous disease. Here, we investigate the determinants of variability in PARP inhibitor distribution as contributors to incomplete drug responses.

We developed a spatially-resolved pipeline using HGSOC patient-derived explants dosed ex vivo with the clinically available PARP inhibitors, olaparib, rucaparib and niraparib. Drug distribution was mapped across tissue sections using mass spec imaging (MSI) and correlated to the whole-transcriptome spatial profile of an adjacent section. To deconvolute the determinants of drug accumulation from responses arising following variable drug exposure, we modelled our findings in vitro, exploiting rucaparib’s fluorescence to separate high- and low-drug cells using FACS and single-cell high-content microscopy, alongside LC-MS for non-fluorescent drugs. Human tissue was obtained from consenting patients under regulatory approval from the Imperial College Healthcare Tissue Bank.

MSI revealed a high degree of inter-patient and intra-tumour heterogeneity in PARP inhibitor accumulation. Spatial transcriptomics demonstrated that regions with high niraparib and rucaparib were significantly enriched for apoptotic and lysosomal signatures. We discovered that these weak base drugs accumulate within lysosomes via pH-dependent partitioning, creating a functional reservoir that maintains elevated nuclear drug levels and enhances DNA damage and cytotoxicity. Additionally, inhibiting lysosomal acidification reduced the bioavailability and efficacy of niraparib and rucaparib but had no impact on olaparib, which lacks basic moieties.

Our results indicate that the uneven distribution of PARP inhibitors may drive therapeutic resistance in HGSOC by creating pockets of low drug exposure, while identifying lysosomal content as a determinant of this heterogeneity. By showing that high-drug cell populations display enhanced drug responses, and that these are linked to lysosomal function, we challenge the traditional view of lysosomes as "drug traps". Instead, our data suggests that for weakly basic PARP inhibitors, lysosomes may function as a bioavailable reservoir that sustains target engagement, opening new pharmacological avenues to enhance their efficacy.

This research was supported by the MRC and Victoria’s Secret Global Fund for Women’s Cancers Career Development Award, in partnership with Pelotonia and AACR. We acknowledge the contributions of G. Zhang, C. Whilding, I. Andrew, A. Montoya, B. Pate, G.R. Young, P. Shliaha, L. Game, B. Lenhard, D. Marks, N. Lambie, K. Tyson, V. Wu, Z. Takats, N. Matthews, and I. McNeish.