The following tissues were also harvest at each time point: liver, lungs, femur (for bone marrow), spleen, heart, stomach, whole small intestine, whole large intestine, mesenteric lymph nodes, kidneys, ovaries, and skeletal muscle (gastrocnemius). likely VEGFC. This complex was trapped within the blood clot during serum preparation from blood, but not within the blood cell pellet during plasma preparation. Low level heparin infusion in mice slowed down clot formation during serum preparation and allowed for better recovery of the radiolabeled antibody in serum. No tissue sink was found in mice. Thus, during this characterization, we determined that the blood Cilostazol sampling matrix greatly impacted the amount of antibody recovered in the samples, therefore, altering its derived pharmacokinetic parameters. Target biology should be considered when selecting appropriate sampling matrices for PK analysis. Keywords:immunocomplex, matrix effects, pharmacokinetics, VEGFC == Abbreviations == baculovirus enzymelinked immunosorbent assay Institutional Animal Care and Use Committee intravenous noncompartmental analysis phosphate buffered solution pharmacokinetics prothrombin time sizeexclusion highperformance liquid chromatography vascular growth factor C == 1. INTRODUCTION == Vascular endothelial growth factorC Cilostazol (VEGFC) is a soluble member of the VEGF family of ligands, also containing VEGFA, VEGFB, VEGFD, VEGFE, and placental growth factor.1The structures among the members are similar with all of them containing the highly conserved VEGF homology domain.2VEGFC is initially synthesized as a prepropeptide that undergoes both intracellular and extracellular processing before achieving its mature form as a homodimer that is held together through noncovalent bonds at a molecular weight ~ 42 kDa.2,3It can interact with VEGF receptor2 (R2) and Cilostazol VEGFR3 to stimulate proliferation and migration of endothelial cells and increases vascular permeability.3VEGFC mediates lymphangiogenesis via VEGFR3 and via a similar angiogenesis signally pathway as VEGFA via VEGFR2.4 VEGFC has wide expression in normal tissues, with measurable mRNA levels in muscle, thyroid, ovary, colon, liver, placenta, and spleen1and measurable protein Mbp levels by immunohistochemistry in breast and prostate.5It has also been shown to be Cilostazol expressed in various tumor tissues as well, such as thyroid, prostate, gastric, colorectal, lung, and breast.6VEGFC increases tumor metastasis7and by decreasing VEGFC expression in mammary tumors in mice, it was possible to decrease spontaneous metastasis and improve survival.6Thus, VEGFC appears to be an important tumor angiogenic target. An antibody was developed against VEGFC and in vivo studies were conducted in order to characterize its pharmacokinetics (PK) across several preclinical species including mouse, rat, and cynomolgus monkey. In all species tested, the antiVEGFC antibody cleared faster than a typical IgG1.8Additionally, in mouse and rat, it had different PK properties depending on whether plasma or serum was collected. The goal of this work was to investigate the factors contributing to this fast clearance and understand the impact of matrix on PK. In vitro studies were performed to determine whether the antibody binds to blood cells, aggregates, degrades, or forms a complex with an endogenous protein. These studies were followed by mouse studies to verify whether the findings in vitro translated to in vivo. Furthermore, a tissue distribution study was conducted to determine whether the antibody distributed specifically to any tissues, which could also affect its Cilostazol in vivo clearance. == 2. EXPERIMENTAL == All animal studies were conducted using protocols approved by each facility’s Institutional Animal Care and Use Committee (IACUC). == 2.1. PK study in athymic nude mice == In all, 18 experimentally nave athymic nude mice (Charles River Laboratories, Hollister, CA) were administered intravenously antiVEGFC either at 1 mg/kg (n = 9) or 10 mg/kg (n = 9). Blood was collected from these mice periodically over 28 days and processed for either plasma (n = 6/group) or serum (n = 3/group). The plasma and serum samples were analyzed for antiVEGFC concentration using an antigen coat ELISA. == 2.2. PK study in Sprague Dawley rats == In all, 18.