Only 0.7% of injected nanoparticles actually reach tumors, and the EPR effect that was supposed to fix this does not work in humans

After three decades and billions of dollars invested in nanomedicine for cancer, a landmark meta-analysis by Wilhelm et al. across 117 preclinical studies found that the median tumor accumulation of administered nanoparticles is just 0.7% of the injected dose. The entire field has been built on the Enhanced Permeability and Retention (EPR) effect — the idea that leaky tumor vasculature lets nanoparticles passively accumulate in tumors. But the EPR effect, while demonstrable in mouse xenograft models, largely fails in human clinical settings. This matters because the entire value proposition of nano-enabled cancer therapeutics rests on improved targeting: deliver the drug to the tumor, spare healthy tissue, reduce side effects. If 99.3% of the injected nanoparticles end up in the liver, spleen, and kidneys instead, then the therapeutic index advantage over conventional chemotherapy shrinks dramatically. Patients still get toxicity. Pharma companies still face Phase III failures. The billions spent on nanoparticle drug delivery platforms yield incremental improvements at best. The problem persists structurally because mouse tumor models — subcutaneous xenografts in immunodeficient mice — have artificially leaky vasculature that exaggerates the EPR effect. Human tumors are heterogeneous, with variable perfusion, dense stroma, and high interstitial fluid pressure that actively resist nanoparticle penetration. But academic incentives reward publishing in these mouse models, and pharma pipelines are built on preclinical data from them. Switching to orthotopic or patient-derived models would be more predictive but slower and more expensive, so the field keeps optimizing nanoparticle properties within a fundamentally broken paradigm. Researchers have called for a moratorium on EPR-based claims, but the institutional momentum is enormous.