Modern drug analysis techniques are an integral part of research and healthcare today. Regrettably, the current approach for sample collection consists of literally removing “a piece of the patient or experimental animal” (whether it is a blood sample or a tissue biopsy) that is sent to the laboratory for analysis. Such a procedure poses ethical issues, health risks for the medical personnel, and inconvenience for the investigated being (patient or lab animal). Furthermore, although it has been recognized that tissue concentrations are more predictive of clinical outcome than plasma concentrations, the assessment of drug distribution and target site pharmacokinetics is very difficult because of lack of appropriate methodology. Point-of-care devices for measuring electrolytes, cardiac markers, and several small molecules now reside in emergency rooms and even at patient bedsides - but there are no clinical devices for fast monitoring of drug levels in body fluids and tissues. The need for such devices is even greater for difficult to treat diseases such as neoplasms. In response to the current challenges faced by drug analysis for pharmacokinetics, pharmacodynamics, and therapeutic drug monitoring, we are proposing to apply innovative microsampling approaches to measure therapeutic antibody concentrations in the tumors without collecting a physical sample from the investigated organism. These methods feature reliable calibration, freedom from pumps, no osmotic effects, and significant improvements in sensitivity. Furthermore, they are faster and more economical than classical ones. The new methods will be applied for the first time to measure the tumor concentration of immune checkpoint inhibitors that act by blocking the programmed cell death receptors on T cells. The proposed research will utilize biocompatible sampling devices for direct microextraction of antibody drugs from tumors. These devices will be prepared by covering flexible medical-grade metal wires with biocompatible polymers and high affinity extractive phases. The extracted analytes will be quantified by chromatography and mass spectrometry to offer high specificity and low limits of detection. For quantitation, several calibration methods will be applied, such as external standard, diffusion-based calibration, and kinetic calibration. The data will be used to build a pharmacokinetic-pharmacodynamic model for a representative therapeutic antibody, such as pembrolizumab. The new in vivo sampling method will be validated by comparison with large pore microdialysis. It is expected that microextraction will provide better sensitivity, accuracy, less tissue damage (due to smaller size), portability, and convenience (no pumps and fluids).