Polysubstance use makes up a majority of drug use, yet relatively few studies investigate the abuse-related effects of drug mixtures. Dose-addition analyses provide a rigorous and quantitative method to determine the nature of the interaction (i.e., supraadditive, additive, or subadditive) between two or more drugs. As briefly reviewed here, studies in rhesus monkeys have applied dose-addition analyses to group level data to characterize the nature of the interaction between the reinforcing effects of stimulants and opioids (e.g., mixtures of cocaine + heroin). Building upon these foundational studies, more recent work has applied dose-addition analyses to better understand the nature of the interaction between caffeine and illicit stimulants such as MDPV and methamphetamine in rats. In addition to utilizing a variety of operant procedures, including drug discrimination, drug self-administration, and drug-primed reinstatement, these studies have incorporated potency and effectiveness ratios as a method for both statistical analysis and visualization of departures from additivity at both the group and individual subject level. As such, dose-addition analyses represent a powerful and underutilized approach to quantify the nature of drug-drug interactions that can be applied to a variety of abuse-related endpoints in order to better understand the behavioral pharmacology of polysubstance use.

“Bath salts” preparations often contain combinations of synthetic cathinones (e.g., 3,4-methylenedioxymethcathinone [methylone], 3,4-methylenedioxypyrovalerone [MDPV]), and caffeine, and evidence suggests that mixtures of synthetic cathinones and caffeine (e.g., MDPV + caffeine or methylone + caffeine) can be more potent and/or effective reinforcers than predicted for an additive interaction.

The recreational use of synthetic cathinones has become a serious public health problem worldwide. In the US, “bath salts” products often contain MDPV (a monoamine transporter inhibitor) or methylone (a monoamine transporter substrate); however, preparations often contain mixtures of multiple synthetic cathinones, or mixtures of a synthetic cathinone and caffeine. In order to determine if the reinforcing effects of mixtures of “bath salts” constituents differed from the reinforcing effects of the constituents alone, adult male Sprague Dawley rats were trained to self‐administer either 0.032 mg/kg/inf MDPV or 0.32 mg/kg/inf methylone under a progressive ratio (PR) schedule of reinforcement. Drug mixtures were based on the concept of dose equivalence, with each pair of drugs evaluated at three ratios (3:1, 1:1, and 1:3) relative to the mean ED50 for each drug. Dose addition analyses were used to calculate predicted effect levels for an additive interaction at each dose pair of each combination of drugs using the Emax, ED50, and slope parameters derived from dose‐response curves obtained in individual rats. Although high levels of responding were maintained by each of the “bath salts” mixtures, the nature of the interaction varied depending upon the constituent drugs. For combinations containing a cathinone and caffeine (MDPV+caffeine and methylone+caffeine), the interactions were generally additive, however, when mixed at a 3:1 ratio of their ED50s the reinforcing effects of both sets of mixtures were found to be supra‐additive. Interestingly, although combining caffeine with MDPV appeared to only increase the potency of the mixture, combining caffeine and methylone significantly increased the reinforcing effectiveness of the a “bath salts” mixture beyond that which would be predicted for an additive interaction. Unlike with combinations of cathinones and caffeine, when MDPV was combined with methylone departures from additivity tended to occur in the sub‐additive direction, with the onset of toxicity observed with larger dose pairs of the MDPV+methylone mixtures. Together, these studies suggest that the reinforcing effects of binary mixtures containing common bath salts constituents (i.e., MDPV, methylone, and caffeine) are generally additive in nature; however, both supra‐additive (MDPV+caffeine and methylone+caffeine) and sub‐additive interactions (MDPV+methylone) can be observed. Although such interactions could account for the high rates of bath salts abuse, further study will be needed to determine if similar interactions exist between these and other bath salts constituents with regard to other abuse‐related and toxic effects. Support or Funding Information This study was supported by a National Institutes of Health (NIH) research grant (R01 DA039146) from the National Institute on Drug Abuse (NIDA), as well as the NIH Intramural Research Programs of NIDA and National Institute on Alcohol Abuse and Alcoholism (NIAAA).

Bath salts use is associated with high rates of abuse, toxicity, and death. Bath salt preparations often contain mixtures of drugs including multiple synthetic cathinones (eg, 3,4-methylenedioxypyrovalerone (MDPV) or 3,4-methylenedioxymethcathinone (methylone)) or synthetic cathinones and caffeine; however, little is known about whether interactions among bath salt constituents contribute to the abuse-related effects of bath salts preparations. This study used male Sprague–Dawley rats responding under a progressive ratio schedule to quantify the reinforcing effectiveness of MDPV, methylone, and caffeine, administered alone and as binary mixtures ( n =12 per mixture). Each mixture was evaluated at four ratios (10 : 1, 3 : 1, 1 : 1, and 1 : 3) relative to the mean ED 50 for each drug alone. Dose-addition analyses were used to determine the predicted, additive effect for each dose pair within each drug mixture. MDPV, methylone, and caffeine maintained responding in a dose-dependent manner, with MDPV being the most potent and effective, and caffeine being the least potent and effective of the three bath salts constituents. High levels of responding were also maintained by each of the bath salts mixtures. Although the nature of the interactions tended toward additivity for most bath salts mixtures, supra-additive (3 : 1 MDPV : caffeine, and 3 : 1 and 1 : 1 methylone : caffeine) and sub-additive (3 : 1, 1 : 1, and 1 : 3 MDPV : methylone) interactions were also observed. Together, these findings demonstrate that the composition of bath salts preparations can have an impact on both their reinforcing potency and effectiveness, and suggest that such interactions among constituent drugs could contribute to the patterns of use and effects reported by human bath salts users.

"Bath salts" preparations contain synthetic cathinones which interact with monoamine transporters and function as either monoamine uptake inhibitors or releasers. 3,4-Methylenedioxypyrovalerone (MDPV), 3,4-methylenedioxymethcathinone (methylone), and 4-methylmethcathinone (mephedrone) were three of the most common cathinones (i.e., "first-generation" cathinones); however, after the US Drug Enforcement Administration placed them under Schedule I regulations, they were replaced with structurally related cathinones that were not subject to regulations (i.e., "second-generation" cathinones). Although the reinforcing effects of some second-generation cathinones have been described (e.g., alpha-pyrrolidinopentiophenone [alpha-PVP]), little is known about how structural modifications, particularly those involving the methylenedioxy moiety and alpha-alkyl side chain, impact the abuse liability of other second-generation cathinones (e.g., alpha-pyrrolidinopropiophenone [alpha-PPP], 3,4-methylenedioxy-alpha-pyrrohdinobutiophenone [MDPBP], and 3,4-methylenedioxy-alpha-pyrrolidinopropiophenone [MDPPP]). The present study used male Sprague-Dawley rats (n = 12 per drug) to directly compare: (1) the acquisition of responding for alpha-PVP (0.032 mg/kg/inf), alpha-PPP (0.32 mg/kg/inf), MDPBP (0.1 mg/kg/inf), and MDPPP (0.32 mg/kg/inf) under a fixed ratio (FR) 1 schedule of reinforcement; and (2) full dose-response curves for each drug to maintain responding under an FR5 schedule of reinforcement. The average number of days (similar to 4 days) and percentage (100%) of rats that acquired self-administration was similar for each drug. The observed rank order potency to maintain responding under an FR5 schedule of reinforcement (alpha-PVP approximate to MDPBP>alpha-PPP > MDPPP) is consistent with their potencies to inhibit dopamine uptake. These are the first studies to report on the reinforcing effects of the unregulated second-generation cathinones MDPBP, MDPPP, and alpha-PPP and indicate all three compounds are readily self-administered, suggesting each possesses high potential for abuse. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'