Flow Rate and Apparent Volume of Cerebrospinal Fluid in Rhesus Macaques (Macaca mulatta) Based on the Pharmacokinetics of Intrathecally Administered Inulin

Cerebrospinal fluid (CSF) flow rate and volume are fundamental to the design and interpretation of preclinical pharmacokinetics and pharmacodynamics studies in NHP. To determine the values of CSF flow rate and volume, we evaluated the plasma and CSF pharmacokinetics of inulin, an inert polysaccharide tracer, in 5 rhesus macaques with CSF ventricular reservoirs and lumbar ports; these reservoirs and ports facilitate humane intrathecal administration and serial CSF sampling in unanesthetized macaques. Inulin was administered intrathecally via the CSF ventricular reservoir (n = 3), followed by the collection of lumbar CSF via the lumbar port and plasma. The contribution of dietary inulin was evaluated by using pre- and postprandial inulin plasma concentrations (n = 2) and a feed analysis of the NHP diet. Inulin concentrations were quantified using ELISA. Pharmacokinetic parameters were calculated by using noncompartmental methods. Daily diet was analyzed for inulin by using Official Method no. 997.08 of AOAC International. In male rhesus macaques, the mean CSF flow rate, established via inulin clearance after IT administration, was 0.018 ± 0.003 mL/min; mean CSF volume, established based on apparent volume of distribution, was 10.17 ± 0.63 mL. In plasma, inulin was quantifiable in all pre-administration samples and increased over the sampling period, precluding interpretation of plasma pharmacokinetics. Evaluation of the effect of diet on plasma concentrations established quantifiable inulin levels that showed minimal variation relative to the prandial state. Analysis of the feed detected 5 inulin types ranging from 1100 to 1440 mg per100 g. The diet was the source of detectable pre-administration inulin plasma concentrations, whereas inulin was not detected in CSF before inulin administration.

Successful treatment of CNS disease with a therapeutic agent requires CNS penetration of the agent across the blood–brain barrier (BBB) to its site of action, to achieve an effective concentration and duration.³⁰ The BBB⁷ limits access into the CNS for systemically administered agents through a complex physical and chemical system.²⁸ In the presence of CNS disease such as malignant glioma, the properties of the BBB may undergo changes permitting greater diffusion of systemically administered agents into the CNS.²⁸ However, the BBB is not static, and permeability changes in the presence of disease may be transient, occur only partially, or not occur.³⁰ Therefore, an agent''s activity against a target demonstrated in vitro, or in vivo in preclinical murine animal models, may be ineffective in patient clinical trials, when the agent fails to reach the target or does not reach the target at an effective concentration or duration^17,27 due to the varying, potentially restrictive, permeability of the BBB in patients. Intrathecal (IT) drug administration is an established alternative administration route that bypasses the BBB and delivers the agent directly into the CSF.²⁹ IT administration is accomplished as intraventricular delivery into the ventricles of the brain or as intralumbar delivery into the spinal column.Efficacious treatment regimens for systemic or IT administration of an agent fundamentally rely on preclinical pharmacokinetics–pharmacodynamics studies that provide information on parameters such as drug plasma and CSF concentrations, duration of measurable drug (pharmacokinetics) or drug activity (pharmacodynamics), drug elimination and distribution, and adverse events.^22,26 Drug exposure, or concentration of a drug over time, is defined by the Area Under the Curve, (AUC). The duration is typically described as the elimination half-life (t_1/2), which is the time required for half of the agent to be biologically reduced quantitatively. Elimination is represented by clearance, which is defined as the rate at which an agent is biologically removed. Distribution, as the apparent volume of distribution (V_d), is the apparent fluid volume required to contain the total amount of drug administered as it relates to the drug concentration in the biologic fluid (plasma, serum, whole blood, or CSF) from which it was measured.¹The interpretation of pharmacokinetics parameters, such as AUC, t_1/2, clearance, and V_d, to develop IT treatment rationales is improved when the species-specific values of CSF flow rate and volume are available for comparison to preclinical pharmacokinetics study results. Uniquely, the values of CSF volume and flow rate are independent of body weight,¹¹ and they serve to establish the potential concentration of drug in the CSF space (exposure and duration) and a mechanism for clearance via either CSF flow or absorption (elimination and distribution).Previously established NHP CSF access models—the CSF ventricular reservoir¹² and lumbar port¹⁶ models (Figure 1)—were developed in combination and used with inulin to evaluate rhesus macaque CSF flow rate and volume. These NHP models, developed in our laboratory, facilitate humane systemic and IT administration (via the CSF ventricular reservoir for the current study) as well as rapid serial CSF collection (via the lumbar port) and plasma collection, via an indwelling femoral intravenous port, in unanesthetized rhesus macaques. Inulin, a plant-based water-soluble polysaccharide, is relatively unaffected by absorption or secretion and is resistant to degradation allowing the substance to be used as a tracer in biologic fluid.^4,19 Because these properties of inulin preclude diffusion across the BBB and tissue absorption, a flow rate and volume can be calculated by using a known administered quantity.Open in a separate windowFigure 1.NHP CSF access models. CSF ventricular (lateral and 4^th) reservoirs and lumbar port.In the current study, we determined the CSF flow rate via clearance and of volume via V_d (hereafter as apparent volume) in rhesus macaques after intraventricular administration of inulin, lumbar CSF collection, and the subsequent quantification and pharmacokinetics analysis of the agent in CSF. Plasma concentrations of inulin were determined also. Because inulin was found to be quantifiable in the plasma prior to intraventricular administration for the pharmacokinetics study, we performed a secondary study to analyze the daily feed as a potential source of inulin in the plasma and to determine the influence of the daily NHP diet on pre- and postprandial plasma inulin levels.