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Open Access Research

Glutamate carboxypeptidase activity in human skin biopsies as a pharmacodynamic marker for clinical studies

Camilo Rojas1, Marigo Stathis1, Michael Polydefkis2, Michelle A Rudek3, Ming Zhao3, Gigi J Ebenezer2 and Barbara S Slusher12*

Author Affiliations

1 Brain Science Institute, Johns Hopkins School of Medicine, 855 North Wolfe Street, Baltimore, MD 21205, USA

2 Department of Neurology, Johns Hopkins School of Medicine, 1550 Orleans Street, Baltimore, MD 21231, USA

3 Department of Oncology, Johns Hopkins School of Medicine, 1650 Orleans Street, Baltimore, MD 21231, USA

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Journal of Translational Medicine 2011, 9:27  doi:10.1186/1479-5876-9-27

Published: 9 March 2011

Abstract

Background

Glutamate excitotoxicity is thought to be involved in the pathogenesis of neurodegenerative disease. One potential source of glutamate is N-acetyl-aspartyl-glutamate (NAAG) which is hydrolyzed to glutamate and N-acetyl-aspartate (NAA) in a reaction catalyzed by glutamate carboxypeptidase (GCP). As a result, GCP inhibition is thought to be beneficial for the treatment of neurodegenerative diseases where excess glutamate is presumed pathogenic. Both pharmacological and genetic inhibition of GCP has shown therapeutic utility in preclinical models and this has led to GCP inhibitors being pursued for the treatment of nervous system disorders in human clinical trials. Specifically, GCP inhibitors are currently being developed for peripheral neuropathy and neuropathic pain. The purpose of this study was to develop a pharmacodynamic (PD) marker assay to use in clinical development. The PD marker will determine the effect of GCP inhibitors on GCP enzymatic activity in human skin as measure of inhibition in peripheral nerve and help predict drug doses required to elicit pharmacologic responses.

Methods

GCP activity was first characterized in both human skin and rat paw pads. GCP activity was then monitored in both rodent paw pads and sciatic nerve from the same animals following peripheral administration of various doses of GCP inhibitor. Significant differences among measurements were determined using two-tailed distribution, equal variance student's t test.

Results

We describe for the first time, a direct and quantifiable assay to evaluate GCP enzymatic activity in human skin biopsy samples. In addition, we show that GCP activity in skin is responsive to pharmacological manipulation; GCP activity in rodent paws was inhibited in a dose response manner following peripheral administration of a potent and selective GCP inhibitor. Inhibition of GCP activity in rat paw pads was shown to correlate to inhibition of GCP activity in peripheral nerve.

Conclusion

Monitoring GCP activity in human skin after administration of GCP inhibitors could be readily used as PD marker in the clinical development of GCP inhibitors. Enzymatic activity provides a simple and direct measurement of GCP activity from tissue samples easily assessable in human subjects.