A Novel Study Using Accelerated Mass Spectrometry to Evaluate the Pharmacokinetics of Total (14)C AL-8309 (Tandospirone) Following Topical Ocular Administration in Healthy Male Subjects.

The primary objective of this study is to characterize the pharmacokinetics of total (14)C concentrations following bilateral, topical ocular drops of (14)C-AL-8309B labeled either at the pyrimidyl ring (cohort A) position or at the imido-carbonyl ring (cohort B) position twice daily from day 1 through day 6 and once in the morning of day 7 in 16 healthy male subjects (8 per cohort). Each drop (approximately 24 µL) of (14)C-AL-8309B 1.75% ophthalmic solution (equivalent to 420 µg-equiv AL-8309) contained approximately 500 nCi of (14)C-AL-8309. AL-8309 systemic absorption was relatively slow; the time of maximum observed plasma concentrations ranged from 0.25 to 3 hours. Moderate accumulation (1.48- to 1.86-fold) was observed in the mean systemic total (14)C plasma concentrations at steady state (day 7) compared with single dose (day 1). The mean total (14)C eliminated was 3.5-fold and 3.7-fold greater in the urine than the feces for cohort A and cohort B, indicating that (14)C-AL-8309 is primarily excreted through renal elimination. Single and multiple topical doses of AL-8309B were found to be safe and well-tolerated in healthy subjects. This is the first reported use of accelerator mass spectrometry technology with a topically applied ophthalmic product.

Carnosine uptake in rat choroid plexus primary cell cultures and choroid plexus whole tissue from PEPT2 null mice.

PEPT2 is functionally active and localized to the apical membrane of rat choroid plexus epithelial cells. However, little is known about the transport mechanisms of endogenous neuropeptides in choroid plexus, and the role of PEPT2 in this process. In the present study, we examined the uptake kinetics of carnosine in rat choroid plexus primary cell cultures and choroid plexus whole tissue from wild-type (PEPT2(+/+)) and null (PEPT2(-/-)) mice. Our results indicate that carnosine is preferentially taken up from the apical as opposed to basolateral membrane of cell monolayers, and that basolateral efflux in limited. Transepithelial flux of carnosine was not distinguishable from that of paracellular diffusion. The apical uptake of carnosine was characterized by a high affinity (K(m) = 34 microM), low capacity (V(max) = 73 pmol/mg protein/min) process, consistent with that of PEPT2. The non-saturable component was small (K(d) = 0.063 microL/mg protein/min) and, under linear conditions, was only 3% of the total uptake. Studies in transgenic mice clearly demonstrated that PEPT2 was responsible for over 90% of carnosine's uptake in choroid plexus whole tissue. These findings elucidate the unique role of PEPT2 in regulating neuropeptide homeostasis at the blood-cerebrospinal fluid interface.

Role of PEPT2 in peptide/mimetic trafficking at the blood-cerebrospinal fluid barrier: studies in rat choroid plexus epithelial cells in primary culture.

Recent studies have established the functional and molecular presence of a high-affinity peptide transporter, PEPT2, in whole tissue rat choroid plexus. However, the precise membrane location and directionality of PEPT2-mediated transport is uncertain at present. In this study, we examined the transport kinetics of a model dipeptide, glycylsarcosine (GlySar), along with the protein expression of PEPT2 using primary cell cultures of choroidal epithelium from neonatal rats. GlySar accumulation and transepithelial transport were 3 to 4 times higher when introduced from the apical as opposed to the basal side of the monolayers. GlySar apical uptake was also stimulated by an inwardly directed proton gradient. The uptake of GlySar was inhibited by di/tripeptides, carnosine, and alpha-amino cephalosporins but was unaffected by amino acids, cephalosporins lacking an alpha-amino group, and organic anions and cations. The Michaelis constant (K(m)) of GlySar was 59.6 microM for apical uptake and 1.4 mM for basal uptake; this is consistent with the high-affinity properties of PEPT2 at the apical membrane. Immunoblot analyses and immunofluorescent confocal microscopy demonstrated the presence of PEPT2, but not PEPT1, in rat choroid plexus epithelial cells. Moreover, PEPT2 was present in the apical and subapical regions of the cell but was absent in the basolateral membrane. These findings demonstrate, for the first time, that PEPT2 protein is present at the apical membrane of choroidal epithelial cells and that it is functionally active at this membrane surface. The results suggest that PEPT2 may have a role in the efflux of peptides and/or mimetics from cerebrospinal fluid to the blood.