Why is it that when given the same chemotherapy drug, some patients see remarkable recovery while others show no response at all? A new study suggests that this discrepancy—often attributed simply to a patient's genetics—may actually depend on exactly where the drug ends up once it enters a cancer cell.
A research team led by Dr. Louise Fets at the UK Medical Research Council Laboratory of Medical Sciences (MRC LMS) has identified a phenomenon where certain chemotherapy drugs become trapped and stored within "lysosomes"—organelles inside tumor cells. This trapping leads to an uneven distribution of the drug throughout the tumor. The findings were recently published in the international journal Nature Communications.
Significant Concentration Gaps Occur Even Within the Same Tumor
While cancer treatments have advanced rapidly, PARP inhibitors still face significant limitations: in some patients, the effects are modest, or the cancer develops resistance over time. For these drugs to be effective, they must accumulate inside cancer cells at a high enough concentration to trigger cell death. However, the precise factors determining how these drugs spread within a tumor have remained unclear until now.
To investigate, researchers used a technique where ovarian cancer tissue obtained from patients was thinly sliced and kept alive in a laboratory setting. They treated these samples with PARP inhibitors and used mass spectrometry imaging to map exactly where the drugs accumulated. By combining this with spatial transcriptomics, they compared gene-expression differences between regions of high and low drug concentration within the same tissue.
The results revealed massive disparities in drug distribution. Even when the same dose was administered, the concentration varied significantly not only between different patients but even within different areas of the same tumor.
Lysosomes Act as a “Hidden Drug Reservoir”
The researchers identified lysosomes—structures responsible for breaking down and recycling waste within a cell—as the primary cause of this uneven spread. Certain PARP inhibitors are drawn into these lysosomes and stored there after entering the cell, preventing them from moving freely to their intended targets.
These lysosomes do not function as simple waste bins; instead, they act like a "delayed-release reservoir," slowly leaking the drug over time. This creates a situation where some cancer cells are overwhelmed by high concentrations while neighboring cells are barely affected.
"We confirmed that drug accumulation varies greatly at the single-cell level, and this appears to occur as the drug stays in lysosomes," explained first author Dr. Carmen Ramirez Moncayo. Notably, not all PARP inhibitors behaved the same way: Rucaparib and niraparib were heavily affected by this trapping, whereas olaparib was found to be relatively less affected.
Paving the Way for Personalized Treatment
PARP inhibitors are currently widely used to treat ovarian, breast, and prostate cancers. This study demonstrates that treatment effectiveness depends not just on whether a drug reaches a tumor, but how it is distributed and stored once it gets there.
"If we understand how drugs are taken up and distributed within cells, we can explain why they work for some patients but not for others," said Dr. Louise Fets. "In the future, we may be able to analyze the molecular characteristics of tumors and design more personalized treatment strategies."
However, because this study was conducted on tissue maintained outside the body (ex vivo), the researchers noted that additional factors in a living patient—such as blood flow and tumor blood-vessel structure—may also influence outcomes. The team plans to use animal models and further patient data to determine how these lysosomal storage mechanisms interact in real clinical settings.
