Modeling the route of administration-based enhancement in the brain delivery of EAB 515, studied by microdialysis

Pharmacokinetics of Intravitreal Bevacizumab (Avastin)

PURPOSE

To describe the pharmacokinetics of 1.25 mg of intravitreal bevacizumab (Avastin).

DESIGN

Experimental animal study.

PARTICIPANTS

Twenty Dutch-belted rabbits.

METHODS

One eye of each of 20 rabbits was injected with 1.25 mg of intravitreal bevacizumab. Both eyes of each of 4 rabbits were enucleated at days 1, 3, 8, 15, and 29. Bevacizumab concentrations were measured in aqueous fluid, whole vitreous, and serum.

MAIN OUTCOME MEASURES

Bevacizumab concentrations in the aqueous, vitreous, and serum.

RESULTS

Whereas vitreous concentrations of bevacizumab declined in a monoexponential fashion with a half-life of 4.32 days, concentrations of >10 microg/ml bevacizumab were maintained in the vitreous humor for 30 days. Bevacizumab concentrations in the aqueous humor of the injected eye reached a peak concentration of 37.7 microg/ml 3 days after drug administration. A maximum serum concentration of 3.3 mug/ml was achieved 8 days after intravitreal injection and the concentration fell below 1 microg/ml 29 days after injection. Elimination of bevacizumab from the aqueous humor and serum paralleled that found in the vitreous humor, with half-life values of 4.88 days and 6.86 days, respectively. Very low concentrations of bevacizumab were detected in the fellow uninjected eye. Concentrations of bevacizumab in the vitreous of the fellow eye varied incrementally, from 0.35 ng/ml at 1 day to 11.17 ng/ml at 4 weeks. Concentrations of bevacizumab in the aqueous humor of the fellow eye reached their peak at 1 week, at 29.4 ng/ml, and declined to 4.56 ng/ml at 4 weeks.

CONCLUSION

The vitreous half-life of 1.25 mg intravitreal bevacizumab is 4.32 days in a rabbit eye. Very small amounts of bevacizumab were detected in the serum and in the fellow uninjected eye.

Distribution of the novel antifolate pemetrexed to the brain

Pemetrexed disodium is a novel antifolate that exhibits potent inhibitory effects on multiple enzymes in folate metabolism. Phase II/III clinical trials have shown that pemetrexed is effective against various solid tumors. Like methotrexate, pemetrexed may be useful in treatment of primary and secondary brain tumors. In this study, we examined the central nervous system (CNS) distribution of pemetrexed and the interaction with an organic anion transport inhibitor indomethacin. Male Wistar rats were administered pemetrexed by either single intravenous bolus or constant intravenous infusion. Unbound pemetrexed in blood and brain was measured by simultaneous arterial blood and frontal cortex microdialysis sampling. In the i.v. bolus experiments, indomethacin was administered by i.v. bolus (10 mg/kg) followed by i.v. infusion (0.1 mg/kg/h) in a crossover manner. In the infusion experiments, the same dose of indomethacin was administered after a steady state was reached for pemetrexed. CNS distributional kinetics was analyzed by compartmental and noncompartmental methods. Both bolus and infusion studies showed that pemetrexed has a limited CNS distribution. The mean area under concentration-time curve (AUC)(brain)/AUC(plasma) ratio of unbound pemetrexed was 0.078 +/- 0.038 in the i.v. bolus study. The pemetrexed steady-state brain-to-plasma unbound concentration ratio after i.v. infusion was 0.106 +/- 0.054. The distributional clearance into the brain was approximately 10% of the clearance out of the brain in both the compartmental and noncompartmental analyses. Indomethacin had no effect on either the brain-to-plasma AUC ratio or the steady-state brain-to-plasma concentration ratio. The distribution of pemetrexed into the brain is limited, and an efflux clearance process, such as an efflux transporter, may be involved.

The use of microdialysis in CNS drug delivery studies. Pharmacokinetic perspectives and results with analgesics and antiepileptics

Microdialysis can give simultaneous information on unbound drug concentration-time profiles in brain extracellular fluid (ECF) and blood, separating the information on blood-brain barrier (BBB) processes from confounding factors such as binding to brain tissue or proteins in blood. This makes microdialysis suitable for studies on CNS drug delivery. It is possible to quantify influx and efflux processes at the BBB in vivo, and to relate brain ECF concentrations to central drug action. The half-life in brain ECF vs. the half-life in blood gives information on rate-limiting steps in drug delivery and elimination from the CNS. Examples are given on microdialysis studies of analgesic and antiepileptic drugs.

Application of microdialysis in pharmacokinetic studies

The objective of this review is to survey the recent literature regarding the various applications of microdialysis in pharmacokinetics. Microdialysis is a relatively new technique for sampling tissue extracellular fluid that is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses various aspects of the technique with regard to its use in pharmacokinetic studies, such as: quantitation of the microdialysis probe relative recovery, interfacing the sampling technique with analytical instrumentation, and consideration of repeated procedures using the microdialysis probe. The remainder of the review is devoted to a survey of the recent literature concerning pharmacokinetic studies that apply the microdialysis sampling technique. While the majority of the pharmacokinetic studies that have utilized microdialysis have been done in the central nervous system, a growing number of applications are being found in a variety of peripheral tissue types, e.g. skin, muscle, adipose, eye, lung, liver, and blood, and these are considered as well. Given the rising interest in this technique, and the ongoing attempts to adapt it to pharmacokinetic studies, it is clear that microdialysis sampling will have an important place in studying drug disposition and metabolism.