Nanoparticle-based therapeutics are being used to treat patients with solid tumors. Whereas nanoparticles have been shown to preferentially accumulate in solid tumors of animal models, there is little evidence to prove that intact nanoparticles localize to solid tumors of humans when systemically administered. Here, tumor and adjacent, nonneoplastic tissue biopsies are obtained through endoscopic capture from patients with gastric, gastroesophageal, or esophageal cancer who are administered the nanoparticle CRLX101. Both the pre-and postdosing tissue samples adjacent to tumors show no definitive evidence of either the nanoparticle or its drug payload (camptothecin, CPT) contained within the nanoparticle. Similar results are obtained from the predosing tumor samples. However, in nine of nine patients that were evaluated, CPT is detected in the tumor tissue collected 24-48 h after CRLX101 administration. For five of these patients, evidence of the intact deposition of CRLX101 nanoparticles in the tumor tissue is obtained. Indications of CPT pharmacodynamics from tumor biomarkers such as carbonic anhydrase IX and topoisomerase I by immunohistochemistry show clear evidence of biological activity from the delivered CPT in the posttreatment tumors.nanomedicine | clinical trial | gastric cancer | tumor targeting | nanoparticles N anoparticle-based experimental therapeutics are being used to deliver a variety of different drug molecules to patients with solid tumors (1). Nanoparticle delivery seeks to improve pharmacokinetic (PK) properties (e.g., enhanced solubility of the drug, increased circulation times), alter biodistribution of the drug molecules to have low amounts of drugs in nontarget tissues and increased amounts in tumors, and enhance pharmacodynamics (PD) (e.g., tunable release of the drug at the site of action in the tumor) to produce enhanced efficacy while simultaneously reducing side effects (and most importantly, introducing no new side effects due to the nanoparticle) in patients. These properties can: (i) enable drug combinations formerly prohibited by toxicity limits, (ii) enable new classes of drug delivery [for example, short interfering RNAs (siRNAs)], and (iii) provide cell-specific targeting within a tumor.Delivery of drugs to solid tumors using nanoparticle technology relies on the enhanced permeability and retention (EPR) effect. The mechanistic data regarding the EPR effect come from animal models, primarily xenografted human tumors in mice. Because these xenografted tumors poorly recapitulate the architecture of true human tumors, there is skepticism about whether or not intact nanoparticles can localize in human tumors. Radiolabeled liposomes have been used to assess tumor accumulation in humans (2, 3). In those studies, the amounts of radioactivity accumulated in tumors did correlate with the number of microvessels measured from nine patient biopsies (3). Increased microvessel density may be an indication of increasing potential for the EPR effect. Also, Davis et al. demonstrated dose-depe...