Pharmacokinetic Correlates of the Effects of a Heroin Vaccine on Heroin Self-Administration in Rats

<div><p>The purpose of this study was to evaluate the effects of a morphine-conjugate vaccine (M-KLH) on the acquisition, maintenance, and reinstatement of heroin self-administration (HSA) in rats, and on heroin and metabolite distribution during heroin administration that approximated the self-administered dosing rate. Vaccination with M-KLH blocked heroin-primed reinstatement of heroin responding. Vaccination also decreased HSA at low heroin unit doses but produced a compensatory increase in heroin self-administration at high unit doses. Vaccination shifted the heroin dose-response curve to the right, indicating reduced heroin potency, and behavioral economic demand curve analysis further confirmed this effect. In a separate experiment heroin was administered at rates simulating heroin exposure during HSA. Heroin and its active metabolites, 6-acetylmorphine (6-AM) and morphine, were retained in plasma and metabolite concentrations were reduced in brain in vaccinated rats compared to controls. Reductions in 6-AM concentrations in brain after vaccination were consistent with the changes in HSA rates accompanying vaccination. These data provide evidence that 6-AM is the principal mediator of heroin reinforcement, and the principal target of the M-KLH vaccine, in this model. While heroin vaccines may have potential as therapies for heroin addiction, high antibody to drug ratios appear to be important for obtaining maximal efficacy.</p></div

Distribution of heroin and its metabolites in plasma and brain after i.v. infusion of 0.125 or 0.0625 mg/kg heroin.

<p>Plasma (A) opioid concentrations (mean ± SD) after the 1^st infusion of heroin. Plasma (B) and brain (C) opioid concentrations (mean ± SD) after the 8^th infusion of 0.125 or 0.0625 mg/kg heroin (cumulative doses of 1.0 and 0.5 mg/kg heroin, respectively). Vaccination increased retention of heroin and its metabolites in plasma after the 1^st as well as after all 8 heroin infusions, at both doses. Vaccination also reduced opioid concentrations in brain after all 8 heroin infusions, though effects were greatest at the lower heroin unit dose. Numbers in parentheses are the percent decrease compared to controls. *p<0.05, **p<0.01, and ***p<0.001 compared to KLH controls. Some drug levels were quite low. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115696#pone.0115696.s001" target="_blank">S1 Table</a> for exact values.</p

Effect of vaccination on acquisition of HSA.

<p>Mean (± SEM) infusions (0.06 mg/kg/inf heroin; left panel) and mean (± SEM) lever presses (right panel) on the active and inactive lever per session during the acquisition phase. Different from KLH, **p<0.01.</p

Effects of vaccination on HSA during dose reduction and reinstatement.

<p>Median total heroin infusions in rats that acquired HSA and completed the dose-reduction protocol (left panel). Rats were trained at a heroin unit dose of 0.06 mg/kg/inf. The heroin dose was sequentially decreased every 5 days to 0.03, 0.01, 0.003 and 0 mg/kg to obtain a dose-response curve. Median infusions compared to KLH *p<0.05, ***p<0.001. Proportion (<i>Χ²</i>) below 0.06 mg/kg compared to M-KLH #p<0.05. Blockade of reinstatement of heroin responding in vaccinated rats (right panel). Mean (± SEM) active lever presses during extinction in vaccinated and control rats after a s.c. priming injection of saline or 0.6 mg/kg heroin. *p<0.05 compared to saline control and #p<0.05 compared to KLH control.</p

Relationship between morphine-specific antibody concentrations and opioid concentrations in brain.

<p>Heroin (A), 6-AM (B), and morphine (C) concentrations in brain after the final (8^th) i.v. infusion of 0.125 or 0.0625 mg/kg heroin in vaccinated rats. KLH data shown as mean ± SD. Deviation not shown for ‘KLH, 0.125 mg/kg/inf heroin’ in the top panel because it was large (32±58 ng/g, mean ± SD).</p

Molar ratios of calculated opioid-specific IgG binding sites to the administered heroin dose.

<p>IgG binding site concentrations are the means for all vaccinated rats in that experiment. Doses listed for the HSA studies are the mean total heroin intake over the 2-hr self-administration session. Doses listed for the drug distribution studies are the total amount of heroin administered over the 1-hr experiment.</p>1<p>Last three HSA sessions at 0.06 mg/kg/inf heroin during FR3.</p>2<p>s.c. administration, all other heroin doses administered i.v.</p><p>Molar ratios of calculated opioid-specific IgG binding sites to the administered heroin dose.</p

Relationship between Adcy5 mRNA expression and oxycodone overall dose intake.

<p>Linear regression of Adcy5 mRNA expression in the Caudate Putamen and oxycodone total dose intake (mg/kg) during the entire self-administration protocol. Total dose was calculated as the number of total number of infusions x infusion dose (0.06 mg/kg/inf). Data include all control (KLH) and vaccinated (OXY-KLH) rats.</p

Acute injection of METH reduces ICSS thresholds.

<p>ICSS thresholds (A) and response latencies (B) (expressed as percent of baseline, mean ± SEM) following s.c. injection of METH (0–0.56 mg/kg) in Experiment 1a. ** Significantly different from 0 mg/kg METH, p < 0.01.</p

MAb7F9 modestly attenuates the ability of acute METH to reverse elevations in ICSS thresholds during METH withdrawal.

<p>ICSS thresholds (A) and response latencies (B) (expressed as percent of baseline, mean ± SEM) in rats administered i.v. PBS or 200 mg/kg mAb immediately following cessation of a chronic METH infusion, followed 20 hr later by s.c. SAL or 0.3 mg/kg METH in Experiment 3. *^, ** Significantly different from SAL-injected animals for that mAb dose, p < 0.05 or 0.01. ^#,mAb 200 + METH significantly different from PBS + METH, p < 0.05.</p

MAb7F9 attenuates the ability of acute METH to reduce baseline ICSS thresholds, ICSS thresholds (A) and response latencies (B) (expressed as percent of baseline, mean ± SEM) following i.v. infusion of vehicle (PBS) or mAb (30, 100, or 200 mg/kg) followed by repeated daily injections of s.c. SAL or 0.3 mg/kg METH in Experiment 2.

<p>** Significantly different from PBS + SAL at that session, p < 0.01. ^{#, ##} 200 mAb + METH significantly different from PBS + METH at that session, p < 0.05 or 0.01. For clarity, significant differences in marginal means between groups are not shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0118787#pone.0118787.g003" target="_blank">Fig. 3A or 3B</a>.</p