Biodistribution of Agmatine to Brain and Spinal Cord Following Systemic Delivery .

Agmatine, an endogenous polyamine, has been shown to reduce chronic pain behaviors in animal models and in patients. This reduction is due to inhibition of the GluN2B subunit of the N-methyl-D-Aspartate receptor (NMDAr) in the central nervous system (CNS). The mechanism of action requires central activity, but the extent to which agmatine crosses biological barriers, such as the blood-brain barrier (BBB) and intestinal epithelium, is incompletely understood. Determination of agmatine distribution is limited by analytical protocols with low sensitivity and/or inefficient preparation. This study validated a novel bioanalytical protocol using HPLC-MS/MS for quantification of agmatine in rat biological matrices. These protocols were then used to determine the plasma pharmacokinetics of agmatine and the extent of distribution to the CNS. Precision and accuracy of the protocol met Food and Drug Administration (FDA) standards in surrogate matrix, as well as in corrected concentrations in appropriate matrices. The protocol also adequately withstood stability and dilution conditions. Upon application of this protocol to pharmacokinetic study, IV agmatine showed a half-life in plasma ranging between 18.9 and 14.9 minutes. Oral administration led to a prolonged plasma half-life (74.4-117 minutes), suggesting flip-flop kinetics, with bioavailability determined to be 29-35%. IV administration led to a rapid increase in agmatine concentration in brain, but a delayed distribution and lower concentrations in spinal cord. However, half-life of agmatine in both tissues is substantially longer than in plasma. These data suggest that agmatine adequately crosses biological barriers in rat and that brain and spinal cord pharmacokinetics can be functionally distinct. Significance Statement Agmatine has been shown to be an effective non-opioid therapy for chronic pain, a significantly unmet medical necessity. Here, using a novel bioanalytical protocol for quantification of agmatine, we present the plasma pharmacokinetics and the first report of agmatine oral bioavailability, as well as variable pharmacokinetics across different CNS tissues. These data provide a distributional rationale for the pharmacological effects of agmatine, as well new evidence for kinetic differences between brain and spinal cord.