Opioid crisis has become a significant problem in the United States, with over 3 million residents having suffered from opioid use disorders (OUD) [1]. OUD can be described as the misappropriation of time and energy spent on obtaining or using opioids, which can interfere with work and personal relationships [2]. As the number of OUD patients steadily increase, so do the number of opioid-related overdose deaths [3]. This significant death toll is distinguished by a 5-fold increase in the number of prescription opioid related deaths from 1999 to 2021, 3-fold increase in heroin related deaths from 2010 to 2021, and 23-fold increase in synthetic opioid related deaths from 2013 to 2021 [4].

Most misused licit and illicit opioids are mu-opioid receptor (MOR) agonists [5], [6]. The MOR is a G protein-coupled receptor (GPCR) that upon stimulation leads to activation of Gi/o, inhibition of adenylyl cyclase (therefore decreased production of cAMP), closing of Ca2+ channels, and positive modulation of inwardly rectifying K+ channels [7], [8], [9], [10], [11]. The overall effect is diminished postsynaptic neuron excitation or release of neurotransmitters. This signaling pathway elicits feelings and/or symptoms of euphoria, analgesia, respiratory depression, and sedation [7], [8], [9], [10], [11].

Many OUD treatments such as opioid maintenance therapies target the MOR (Fig. 1) [12]. The pharmacological aims of maintenance therapies are detoxification, alleviation of withdrawal symptoms, and prevention of relapse and overdose [12], [13], [14]. Examples include methadone and buprenorphine (full and partial MOR agonists respectively) which attenuate withdrawal symptoms that OUD patients may experience during detoxification [15], [16], [17], [18], [19]. Their major drawback is poor adherence, high rates of relapse, and potential for misuse and diversion [15], [16], [17], [18], [19]. Another form of maintenance therapy, naltrexone (NTX), is a MOR antagonist that prevents relapse and drug craving [20], [21] The attraction for an MOR antagonist is the reduced risk for misuse, as well as the ability to reduce positive reinforcement with opioid use should a patient relapse [20], [21]. However, a limitation of NTX is the severe withdrawal effects that precipitate in opioid-dependent patients which minimizes patient compliance [22], [23]. This drawback creates a gap for exploration into the design of novel therapeutics to treat OUD patients that minimize opioid-related relapse and overdoses. A selective MOR modulator (low efficacy partial agonist or antagonist) that does not elicit severe side effects in opioid-dependent patients is desired to improve the patient experience.

Employing the ‘message-address’ concept, our drug design and discovery efforts have resulted in creating a diverse collection of small molecules with various substituents on the epoxymorphinan framework [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. Among these, NAT was a lead compound that our group discovered which potently antagonize morphine-mediated antinociception (AD50: 0.42 mg/kg). It successfully penetrated the blood brain barrier and showed significantly high blood/plasma ratio which increased over time. In addition, NAT produced significantly fewer withdrawal symptoms than naloxone (NLX) at equivalent doses [24]. However, a major concern for NAT was related to the thiophene moiety, which is a site for potential oxidative bioactivation. CYP450s can oxidize the thiophene ring to form electrophilic species that react with various proteins and nucleic acids [42]. Reports have shown that substitution at the α-carbon on the thiophene ring may decrease the formation of these toxic metabolites [42]. Thus, in our current work we focused on the design, synthesis, and in vitro/in vivo pharmacological evaluations of several NAT analogs with substitutions at the α-carbon of thiophene in order to identify analogs that are more or at least equipotent compared to NAT.