In central nervous system drug discovery, cerebrospinal fluid (CSF) drug concentration (C(CSF)) has been widely used as a surrogate for unbound brain concentrations (C(u,brain)). However, previous rodent studies demonstrated that when drugs undergo active efflux by transporters, such as P-glycoprotein (P-gp), at the blood-brain barrier, the C(CSF) overestimates the corresponding C(u,brain). To investigate the utility of C(CSF) as a surrogate for interstitial fluid (ISF) concentration (C(ISF)) in nonhuman primates, this study simultaneously determined the C(CSF) and C(ISF) of 12 compounds, including P-gp substrates, under steady-state conditions in cynomolgus monkeys using intracerebral microdialysis coupled with cisternal CSF sampling. Unbound plasma concentrations of non- or weak P-gp substrates were within 2.2-fold of the C(ISF) or C(CSF), whereas typical P-gp substrates (risperidone, verapamil, desloratadine, and quinidine) showed ISF-to-plasma unbound (K(p,uu,ISF)) and CSF-to-plasma unbound concentration ratios (K(p,uu,CSF)) that were appreciably lower than unity. Although the K(p,uu,CSF) of quinidine, verapamil, and desloratadine showed a trend of overestimating the K(p,uu,ISF), K(p,uu,CSF) showed a good agreement with K(p,uu,ISF) within 3-fold variations for all compounds examined. C(u,brain) of some basic compounds, as determined using brain homogenates, overestimated the C(ISF) and C(CSF). Therefore, C(CSF) could be used as a surrogate for C(ISF) in nonhuman primates.

Overall, these data support published Phase III data demonstrating the efficacy of PER as adjunctive therapy for the treatment of refractory partial-onset seizures in patients aged ≥ 12 years.

In drug discovery, the cerebrospinal fluid (CSF) drug concentration (CCSF) has been used as a surrogate for the interstitial fluid (ISF) concentration (CISF). However, the CCSF-to-CISF gradient suggested for P-glycoprotein (P-gp) substrates in rodents causes uncertainty in CISF estimations and subsequent pharmacokinetic-pharmacodynamic analyses. To evaluate the utility of CCSF as a surrogate for CISF, this study directly compared the CCSF with the CISF of 12 compounds, including P-gp substrates, under steady-state conditions in wild-type and Mdr1a(-/-) rats using microdialysis coupled with cisternal CSF sampling. In wild-type rats, the ISF-to-unbound plasma (Kp,uu,ISF) and CSF-to-unbound plasma (Kp,uu,CSF) concentration ratios of the P-gp substrates, except for metoclopramide, were lower than those of the non-P-gp substrates, and the Kp,uu,CSF values were within or close to 3-fold of the Kp,uu,ISF values for all the compounds examined. The Kp,uu,CSF values of the selected P-gp substrates increased in Mdr1a(-/-) rats with a similar magnitude to the Kp,uu,ISF values, resulting in the Kp,uu,CSF-to-Kp,uu,ISF ratios being unchanged. These results suggested that P-gp-mediated active efflux at the blood-brain barrier is a major determinant not only for CISF, but also for CCSF, and that CCSF can be used as a surrogate for CISF even for P-gp substrates in rats.

Estimation of unbound drug concentration in the brain (C u,brain) is an essential part of central nervous system (CNS) drug development. As a surrogate for C u,brain in humans and nonhuman primates, drug concentration in the cerebrospinal fluid (C CSF) collected by lumbar puncture is often used; however, the predictability of C u,brain by lumbar C CSF is unclear, particularly for substrates of the active efflux transporter P-glycoprotein (P-gp). Here, we measured lumbar C CSF in cynomolgus monkey after single intravenous administration of 10 test compounds with varying P-gp transport activities. The in vivo lumbar CSF-to-plasma unbound drug concentration ratios (K p,uu,lumbar CSF) of nonsubstrates or weak substrates of P-gp were in the range 0.885-1.34, whereas those of good substrates of P-gp were in the range 0.195-0.458, and were strongly negatively correlated with in vitro P-gp transport activity. Moreover, concomitant treatment with a P-gp inhibitor, zosuquidar, increased the K p,uu,lumbar CSF values of the good P-gp substrates, indicating that P-gp-mediated active efflux contributed to the low K p,uu,lumbar CSF values of these compounds. Compared with the drug concentrations in the cisternal CSF and interstitial fluid (ISF) that we previously determined in cynomolgus monkeys, the lumbar C CSF were more than triple for 2 and all of the good P-gp substrates examined, respectively. Although lumbar C CSF may overestimate cisternal CSF and ISF concentrations of good P-gp substrates, lumbar C CSF allowed discrimination of good P-gp substrates from the weak and nonsubstrates and can be used to estimate the impact of P-gp-mediated active efflux on drug CNS penetration. Significance Statement This is the first study to systematically evaluate the penetration of various P-gp substrates into lumbar CSF in nonhuman primates. Lumbar CSF may contain >3-fold higher concentrations of good P-gp substrates than ISF and cisternal CSF, but was able to discriminate the good substrates from the weak or nonsubstrates. Because lumbar CSF is more accessible than ISF and cisternal CSF in This article has not been copyedited and formatted. The final version may differ from this version.