Evaluation of a Dual Fentanyl/Heroin Vaccine on the Antinociceptive and Reinforcing Effects of a Fentanyl/Heroin Mixture in Male and Female Rats

Abstract

Opioid-targeted vaccines represent an emerging treatment strategy for opioid use disorder. To determine whether concurrent vaccination against two commonly abused opioids (fentanyl and heroin) would confer broader spectrum opioid coverage, the current study evaluated dual fentanyl/heroin conjugate vaccine effectiveness using a warm water tail-withdrawal and a fentanyl/heroin-vs-food choice procedure in male and female rats across a 105-day observation period. Vaccine administration generated titers of high-affinity antibodies to both fentanyl and heroin sufficient to decrease the antinociceptive potency of fentanyl (25-fold), heroin (4.6-fold), and a 1:27 fentanyl/heroin mixture (7.5-fold). Vaccination did not alter the antinociceptive potency of the structurally dissimilar opioid agonist methadone. For comparison, continuous treatment with a naltrexone dose (0.032 mg/kg/h) shown previously to produce clinically relevant plasma-naltrexone levels decreased the antinociceptive potency of fentanyl, heroin, and the 1:27 fentanyl/heroin mixture by approximately 20-fold. Naltrexone treatment also shifted the potency of 1:27 fentanyl/heroin mixture in a drug-vs-food choice self-administration procedure 4.3-fold. In contrast, vaccination did not attenuate 1:27 fentanyl/heroin mixture self-administration in the drug-vs-food choice procedure. These data demonstrate that a vaccine can simultaneously attenuate the thermal antinociceptive effects of two structurally dissimilar opioids. However, the vaccine did not attenuate fentanyl/heroin mixture self-administration, suggesting a greater magnitude of vaccine responsiveness is required to decrease opioid reinforcement relative to antinociception.

Keywords: antinociception; choice; drug self-administration; fentanyl; heroin; vaccine.

Figure 1.

Panel A shows the relative potency of fentanyl (upward-facing triangles) and heroin (downward-facing triangles) to produce thermal antinociception in the 12 rats of Cohorts 1 and 2 (6 females and 6 males). Both fentanyl and heroin produced “full” antinociceptive effects (i.e., 20 s of tail immersion) in each rat. Depicted are data points that span the 20–80% maximum possible effect level, as these data constitute the linear portion of the dose–effect function and were included in the linear regression analysis. Abscissa: Cumulative IV fentanyl or heroin dose in micrograms per kilogram. Ordinate: %MPE collected in a warm water tail-withdrawal procedure. Points represent mean ± SEM. The 14-day cycle of experimental conditions completed by Cohorts 1 and 2 are depicted in panel B. Panel C shows the timeline of the vaccination and blood collection schedule.

Figure 2.

Panels A–D show opioid antinociceptive potency at baseline (open symbols), during continuous naltrexone (0.032 mg/kg/h, SC) treatment (gray-filled symbols), and at time of maximal fentanyl/heroin vaccine effect (black-filled symbols) for fentanyl, heroin, a 1:27 fentanyl heroin mixture, and methadone, respectively. With the exception of the “methadone + naltrexone” experiment, each opioid produced “full” antinociceptive effects (i.e., 20 s of tail immersion) in each rat. Depicted are data points that span the 20–80% maximum possible effect level, as these data constitute the linear portion of the dose–effect function and were included in the linear regression analysis. Abscissae: Cumulative IV opioid dose in μg/kg. Ordinates: %MPE collected in a warm water tail-withdrawal procedure. Points represent mean ± SEM for the six rats in Cohort 1 (3 females and 3 males).

Figure 3.

Effectiveness of a dual fentanyl/heroin vaccine, continuous naltrexone (0.032 mg/kg/h, SC), or saline substitution for fentanyl and/or heroin to attenuate IV fentanyl/heroin-vs-food choice. Abscissae: unit IV fentanyl/heroin dose in μg/kg (note: fentanyl and/or heroin absent in panels C and F depending on saline substitution condition). Top row ordinates: percentage of completed ratio requirements on the fentanyl/heroin-associated lever. Bottom row ordinates: number of choices completed per component. Panels A and D show selected time points following administration of a dual fentanyl/heroin vaccine in six Cohort 2 rats (3 female and 3 male), although one male rat lost catheter patency prior to the Days 105–108 evaluation and was excluded from that time point. Panels B and E show the last 3 days of 5-day continuous naltrexone (0.032 mg/kg/h, SC) treatment effects in six Cohort 3 rats (3 female and 3 male). Panels C and F show the last 3 days of 5-day saline substitution experiments that included the removal of fentanyl (upward-facing triangles), heroin (downward-facing triangles), or both fentanyl and heroin (saline substitution; squares) in six Cohort 3 rats (3 female and 3 male). Points represent mean ± SEM, and filled symbols denote significant difference relative to baseline, defined as p < 0.05.

Figure 4.

Time course of vaccine effectiveness, antibody midpoint titer levels, and affinities. Abscissae: experimental timeline with arrows indicating vaccination dates. Top ordinates: fold shift in antinociceptive potency of the 1:27 fentanyl/heroin mixture relative to prevaccination ED₅₀ values. Middle ordinates: midpoint titer levels. Bottom ordinates: antibody IC₅₀ values in nM determined from antiserum. Panel A shows vaccine effects on antinociceptive potency as a function of experimental day in Cohort 1 (n = 3 male, 3 female rats) and Cohort 2 (n = 3 male, 3 female rats). Panel B shows vaccine effects on antinociceptive potency as a function of experimental day in 12 rats (6 female and 6 male) from both Cohorts 1 and 2. Panels C and D show antifentanyl (left) and antiheroin (right) midpoint titers as a function of experimental day in Cohort 1 and Cohort 2. Panels E and F show antifentanyl (left) and anti-6-acetylmorphine (active heroin metabolite; right) antibody affinity (IC₅₀ value) as a function of experimental day in Cohort 1 and Cohort 2. All points represent mean ± SEM. One male of Cohort 2 lost catheter patency before the Day 98 evaluation and was excluded from that time point. The asterisk in Panel D indicates significant difference in heroin midpoint titer levels between Cohorts 1 and 2 on Day 56, defined as p < 0.05.