Pharmacokinetics and immunologic consequences of repeated administrations of purified heterologous and homologous butyrylcholinesterase in mice

Abstract

Aim

To assess the consequences of repeated administrations of purified human serum butyrylcholinesterase (Hu BChE) and mouse serum (Mo) BChE into mice.

Main methods

Purified Hu BChE and Mo BChE isolated from the sera of CD-1 mice were administered into Balb/c or CD-1 mice. The enzymes were delivered by i.m. injections of ∼ 100 U (0.15 mg) on day 1 and on day 28, respectively. The effects of two injections were monitored by following blood BChE and anti-BChE IgG levels.

Key findings

Hu BChE displayed a mean residence time (MRT) of 50 h, and an area under the curve (AUC) of 1220 U/ml·h in Balb/c or CD-1 mice. Mo BChE exhibited an MRT of 78 h and an AUC of 1815 U/ml·h in Balb/c mice; the AUC increased to 2504 U/ml·h in CD-1 mice. A second injection of Hu BChE in both strains exhibited a marked reduction in circulatory stability. The circulatory stability of the second injection of Mo BChE was reduced in Balb/c mice, but was almost identical to the first injection in CD-1 mice. Consistent with these observations, circulating anti-BChE IgGs were observed in mice injected with Hu BChE; low levels of anti-BChE IgGs were observed only in Balb/c mice injected with Mo BChE. No antibody response was detected in CD-1 mice following either injection of homologous Mo BChE.

Significance

The identical pharmacokinetic profiles and the absence of an immunologic response following a second administration of homologous BChE support the development of Hu BChE as a detoxifying drug in humans.

Introduction

The exogenous administration of plasma-derived cholinesterases (ChEs) has been successfully used as a safe and efficacious prophylactic treatment to prevent poisoning by organophosphorus compounds (OPs, Doctor et al. 2001). The protection displayed was not only against mortality but also against the adverse physiological and behavioral effects of nerve agent exposure. Of the ChEs evaluated so far, ‘self’ Hu BChE is currently the most suitable candidate for human use. It provides a broad range of protection against the toxicity of all OP nerve agents, displays high bioavailability and long-lasting stability in human circulation, and is expected to be devoid of any adverse immunological responses upon repeated administration into humans (Ashani 2000). In addition to its use as a prophylactic for OP nerve agent exposure, it also has potential use for treating pesticide overexposure, cocaine overdose, or succinylcholine-induced apnea (Ashani 2000).

In most studies, the safety and efficacy of Hu BChE and other ChEs were evaluated following the administration of a single dose of enzyme. However, multiple prophylactic treatments aimed at maintaining long-lasting protective levels of circulating enzyme may be needed to counteract the toxicity of multiple exposures to OPs. Therefore, several studies in the 90's addressed the immunologic effects of repeated exposure of rats, rabbits, and rhesus monkeys to ChEs (Genovese et al., 1993, Gentry et al., 1993, Gentry et al., 1996, Matzke et al., 1999). In all cases, the heterologous enzyme injections resulted in an induction of anti-enzyme antibodies, which neutralized subsequent enzyme injections. On the other hand, two administrations of homologous monkey (Ma) BChE purified from the plasma of pigtailed macaques into pigtailed macaques resulted in a much longer mean residence time (MRT), and no antibody response was detected in macaques following either injection of enzyme (Rosenberg et al. 2002). However, in this study, the pharmacokinetics and immunological consequences of two injections of Hu BChE were not evaluated.

Since mice are routinely used for conducting efficacy studies with ChEs including Hu BChE (Doctor et al. 2001), here we examined the consequences of repeated injections of Hu BChE (heterologous) and Mo BChE isolated from the sera of CD-1 mice (homologous) in mice following two i.m. injections of ∼ 100 U (0.15 mg) on day 1 and on day 28, respectively. This dose is similar to a dose of Hu BChE (200 mg) in humans that can protect from an exposure of up to 2 × LD₅₀ of soman (Ashani and Pistinner 2004). The effects of two injections were monitored by following blood BChE and anti-BChE IgG levels. The rate of elimination of Hu BChE following the first injection was faster compared to that of Mo BChE. The second injection of Hu BChE cleared much faster as compared to the first injection in both Balb/c and CD-1 mice. The second injection of Mo BChE displayed reduced circulatory stability in Balb/c mice, but its stability was almost identical to the first injection in CD-1 mice. Consistent with these observations, circulating anti-BChE IgGs were observed in both Balb/c and CD-1 mice injected with Hu BChE; anti-BChE IgGs were observed only in Balb/c mice injected with Mo BChE. No antibody response was detected in CD-1 mice following either injection of homologous Mo BChE. The results of this study in conjunction with the macaque study (Rosenberg et al. 2002) support the development of Hu BChE as a detoxifying drug in humans.