ABSTRACT
Drug concentrations in cerebrospinal fluid (CSF) are typically used as a as a surrogate measure of their availability in the CNS, and CSF penetration in animal studies are used for assessment of CNS drug delivery in early preclinical drug development. The minipig is a valid alternative to dogs and non-human primates as non-rodent species in preclinical research, but this species presents anatomical peculiarities that make the serial collection of CSF technically challenging. A minimally-invasive serial cerebrospinal fluid collection model via catheterization of the subarachnoid space in conscious minipigs was developed allowing assessment of longitudinal drug pharmacokinetics in the central nervous system in preclinical research. Shortly, the subarachnoid space was accessed in the anesthetized minipig by puncture with a Tuohy needle; when CSF was flowing through the needle a catheter was advanced and thereafter tunneled and fixed on the back. The PK of peptide A administered subcutaneously was performed and CSF could be sampled in the conscious animals for up to 48 h. When compared to the plasma kinetic data, there was a clear difference in the elimination phase of Pept. A from CSF, with an apparent longer average terminal half-life in CSF. The 3Rs are addressed by reducing the number of animals needed for a pharmacokinetic profile in central nervous system and by improving the validity of the model avoiding biases due to anesthesia, blood contamination, and inter-individual variability.
ABSTRACT
OBJECTIVE: 1) To determine the pharmacokinetics of tramadol hydrochloride and its active metabolite, O-desmethyltramadol (M1), after administration through different routes in female and male C57Bl/6 mice; 2) to evaluate the stability of tramadol solutions; and 3) to identify a suitable dose regimen for prospective clinical analgesia in B6 mice. STUDY DESIGN: Prospective, randomized, blinded, parallel design. ANIMALS: A total of 18 male and 18 female C57Bl/6 mice (20-30 g). METHODS: Mice were administered 25 mg kg-1 tramadol as a bolus [intravenously (IV), intraperitoneally (IP), subcutaneously (SQ), orally per gavage (OSgavage)] over 25 hours [orally in drinking water (OSwater) or Syrspend SF (OSSyrsp)]. Venous blood was sampled at six predetermined time points over 4 to 31 hours, depending on administration route, to determine tramadol and M1 plasma concentrations (liquid chromatography and tandem mass spectrometry detection). Pharmacokinetic parameters were described using a noncompartmental model. The stability of tramadol in water (acidified and untreated) and Syrspend SF (0.20 mg mL-1) at ambient conditions for 1 week was evaluated. RESULTS: After all administration routes, Cmax was >100 ng mL-1 for tramadol and >40 ng mL-1 for M1 (reported analgesic ranges in man) followed by short half-lives (2-6 hours). The mean tramadol plasma concentration after self-administration remained >100 ng mL-1 throughout consumption time. M1 was found in the OSSyrs group only at 7 hours, whereas it was detectable in OSwater throughout administration. Tramadol had low oral bioavailability (26%). Short-lasting side effects were observed only after IV administration. Water and Syrspend SF solutions were stable for 1 week. CONCLUSIONS AND CLINICAL RELEVANCE: 1) At the dose administered, high plasma concentrations of tramadol and M1 were obtained, with half-life depending on the administration route. 2) Plasma levels were stable over self-consumption time. 3) Solutions were stable for 1 week at ambient conditions.
