Toxicological characterisation of two novel selective aryl hydrocarbon receptor modulators in Sprague-Dawley rats

Abstract

The aryl hydrocarbon receptor (AHR) mediates the toxicity of dioxins, but also plays important physiological roles. Selective AHR modulators, which elicit some effects imparted by this receptor without causing the marked toxicity of dioxins, are presently under intense scrutiny. Two novel such compounds are IMA-08401 (N-acetyl-N-phenyl-4-acetoxy-5-chloro-1,2-dihydro-1-methyl-2-oxo-quinoline-3-carboxamide) and IMA-07101 (N-acetyl-N-(4-trifluoromethylphenyl)-4-acetoxy-1,2-dihydro-5-methoxy-1-methyl-2-oxo-quinoline-3-carboxamide). They represent, as diacetyl prodrugs, AHR-active metabolites of the drug compounds laquinimod and tasquinimod, respectively, which are intended for the treatment of autoimmune diseases and cancer. Here, we toxicologically assessed the novel compounds in Sprague-Dawley rats, after a single dose (8.75-92.5mg/kg) and 5-day repeated dosing at the highest doses achievable (IMA-08401: 100mg/kg/day; and IMA-07101: 75mg/kg/day). There were no overt clinical signs of toxicity, but body weight gain was marginally retarded, and the treatments induced minimal hepatic extramedullary haematopoiesis. Further, both the absolute and relative weights of the thymus were significantly decreased. Cyp1a1 gene expression was substantially increased in all tissues examined. The hepatic induction profile of other AHR battery genes was distinct from that caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The only marked alterations in serum clinical chemistry variables were a reduction in triglycerides and an increase in 3-hydroxybutyrate. Liver and kidney retinol and retinyl palmitate concentrations were affected largely in the same manner as reported for TCDD. In vitro, the novel compounds activated CYP1A1 effectively in H4IIE cells. Altogether, these novel compounds appear to act as potent activators of the AHR, but lack some major characteristic toxicities of dioxins. They therefore represent promising new selective AHR modulators.

Transcriptomic Impact of IMA-08401, a Novel AHR Agonist Resembling Laquinimod, on Rat Liver

“…In the subacute study, rats received daily either 100 mg/kg C2 dissolved in PEG-400 or vehicle alone for five consecutive days, and were euthanized after a recovery period of five days on experimental day 10. As liver was previously shown to be susceptible to C2-induced Cyp1a1 mRNA abundance [42], and is one of the primary sites of TCDD-mediated toxicities [30,40], hepatic tissue was excised for transcriptomic profiling. For comparison of the transcriptomic impacts of C2 and TCDD, data previously generated from hepatic tissue from TCDD-sensitive Long–Evans ( Turku/AB ; L–E) and TCDD-resistant H/W rats, following a single exposure to 100 µg/kg TCDD and collected at 19 h or 10 days afterwards were used [44,45].…”

“…However, as these compounds act through ligand activation of the AHR, concerns regarding their safety exist. These concerns revolve around the similar structure and mechanism of action of these compounds to those of the most potent AHR-ligands—dioxins, and in particular, TCDD [42]. Our recent in vitro studies with the active derivative of C2 proved it to have virtually equal potency in inducing CYP1A1 activity to TCDD in a rat hepatoma cell line and exhibit similar modelled binding properties to the ligand binding region of the AHR [61].…”

“…LAQ is metabolized partly by N-dealkylation to give the potent AHR agonist DELAQ (deethylated LAQ; IMA-06201) in minute amounts [41]. The prodrug IMA-08401 (henceforth referred to as C2) is a diacetyl prodrug of DELAQ, representing a novel selective AHR modulator designed to effectively hydrolyze to DELAQ in vivo (to a far greater extent than LAQ) and being a potent agonist of the AHR [42]. Rats exposed to C2 demonstrated increased mRNA abundance of multiple ‘AHR-core’ genes across multiple tissue types, similar to that observed following exposure to TCDD [42].…”

“…The prodrug IMA-08401 (henceforth referred to as C2) is a diacetyl prodrug of DELAQ, representing a novel selective AHR modulator designed to effectively hydrolyze to DELAQ in vivo (to a far greater extent than LAQ) and being a potent agonist of the AHR [42]. Rats exposed to C2 demonstrated increased mRNA abundance of multiple ‘AHR-core’ genes across multiple tissue types, similar to that observed following exposure to TCDD [42]. Interestingly, exposed animals did not exhibit most of the major signs of toxicity typically observed with TCDD exposure, despite repeated daily administration of high C2 doses (highest achievable given the solubility characteristics of the compound).…”

“…Interestingly, exposed animals did not exhibit most of the major signs of toxicity typically observed with TCDD exposure, despite repeated daily administration of high C2 doses (highest achievable given the solubility characteristics of the compound). Perhaps most strikingly, these rats did not experience the dramatic body weight loss associated with TCDD-induced wasting syndrome [42,43].…”