Family of Structurally Related Bioconjugates Yields Antibodies with Differential Selectivity against Ketamine and 6-Hydroxynorketamine

The dissociative-hypnotic compound ketamine is being used in an increasingly wide range of therapeutic contexts, including anesthesia, adjunctive analgesia, treatment-resistant depression, but it also continues to be a notable substance of abuse. No specific antidotes exist for ketamine intoxication or overdose. Immunopharmacotherapy has demonstrated the ability to offer overdose protection through production of highly specific antibodies that prevent psychoactive drug penetration across the blood-brain barrier, although antiketamine antibodies have not yet been assessed or optimized for use in this approach. Moreover, generation of specific antibodies also provides an opportunity to address the role of 6-hydroxynorketamine metabolites in ketamine’s rapid-acting antidepressant effect through selective restriction of metabolite access to the central nervous system. Hapten design is a critical element for tuning immune recognition of small molecules, as it affects the presentation of the target antigen and thus the quality and selectivity of the response. Here, we report the synthesis and optimization of carrier protein and conjugation conditions for an initial hapten, norketamine-N-COOH (NK-N-COOH), to optimize vaccination conditions and assess the functional consequences of such vaccination on ketamine-induced behavioral alterations occurring at dissociative-like (50 mg/kg) doses. Iterating from this initial approach, two additional haptens, ketamine-N-COOH (KET-N-COOH) and 6-hydroxynorketamine-N-COOH (HNK-N-COOH), were synthesized to target either ketamine or 6-hydroxynorketamine with greater selectivity. The ability of these haptens to generate antiketamine, antinorketamine, and anti-6-hydroxynorketamine immune responses in mice was then assessed using enzyme-linked immunosorbent assay (ELISA) and competitive surface plasmon resonance (SPR) methods. All three haptens provoked immune responses in vivo, although the KET-N-COOH and 6-HNK-N-COOH haptens yielded antibodies with 5- to 10-fold improvements in affinity for ketamine and/or 6-hydroxynorketamine, as compared to NK-N-COOH. Regarding selectivity, vaccines bearing a KET-N-COOH hapten yielded an antibody response with approximately equivalent Kd values against ketamine (86.4 ± 3.2 nM) and 6-hydroxynorketamine (74.1 ± 7.8 nM) and a 90-fold weaker Kd against norketamine. Contrastingly, 6-HNK-N-COOH generated the highest affinity and most selective antibody profile, with a 38.3 ± 4.7 nM IC50 against 6-hydroxynorketamine; Kd values for ketamine and norketamine were 33- to 105-fold weaker, at 1290 ± 281.5 and 3971 ± 2175 nM, respectively. Overall, these findings support the use of rational hapten design to generate antibodies capable of distinguishing between structurally related, yet mechanistically distinct, compounds arising from the same precursor molecule. As applied to the production of the first-reported anti-6-hydroxynorketamine antibodies to date, this approach demonstrates a promising path forward for identifying the individual and combinatorial roles of ketamine and its metabolites in supporting rewarding effects and/or rapid-acting antidepressant activity.