Emvododstat (PTC299; 4-chlorophenyl 6-chloro-1-[4-methoxyphenyl]-1,3, 4,9-tetrahydro-2H-pyrido[3,4-b]indole-2-carboxylate) is an orally available inhibitor of the de novo pyrimidine synthesis enzyme dihydroorotate dehydrogenase (DHODH) [1, 2]. Inhibition of DHODH results in the depletion of pyrimidine nucleotides in rapidly dividing cells, leading to arrest of cell growth and DNA replication (G1/S-phase cell cycle arrest) and subsequent differentiation or cell death [3]. Cells may utilize salvage pathways whereby pyrimidine nucleotides are synthesized from intermediates formed from the degradation of RNA and DNA taken into the cell [2, 4, 5]. Normal cells typically do not rely on de novo pyrimidine biosynthesis and utilize the salvage pathway to provide pyrimidine nucleotides. Rapidly dividing cells rely on a de novo pathway [2]. Consistent with this, emvododstat does not impair the viability or proliferation of cultured human bone marrow progenitor cells and has shown no evidence of myelotoxicity in toxicity studies in rats and dogs dosed for up to 28 days or in humans [2].

Inhibition of de novo pyrimidine biosynthesis has been identified as a mechanism to inhibit viral replication, including replication of the RNA virus severe acute respiratory syndrome corona virus 2 (SARS-CoV-2) [6, 7]. Further, lethality associated with SARS-CoV-2 infection has been associated with excessive activation of the immune response [7]. Emvododstat has a dual mechanism of action, which inhibits SARS-CoV-2 replication and attenuates the production of pro-inflammatory cytokines [2]. In vitro, cell culture assays indicate a decreased capsid protein (EC50 1.96 nM) and decreased viral titer (EC50 2.6 nM, EC90 53 nM) of SARS-CoV-2019 following cell exposure to emvododstat [2]. This dual mechanism of action together with a well-established safety profile as demonstrated in previous completed clinical studies [8] supports an appropriate benefit/risk profile for evaluating the drug in subjects with coronavirus disease 2019 (COVID-19) with a dose regimen of 200 mg BID for 7 days followed by 50 mg QD for another 7 days.

In vitro CYP inhibition potential was assessed in human liver microsomes for emvododstat and its primary O-desmethyl metabolite up to the maximum solubility for both. Emvododstat was identified as a non-competitive inhibitor of CYP2D6 with an inhibition constant (Ki) of 37.7 nM, indicating a need for clinical confirmation [9]. The purpose of the current study was to investigate potential drug-drug interactions (DDIs) between emvododstat and other medications that are metabolized by CYP2D6. Dextromethorphan is a CYP2D6 substrate and is recognized as a CYP2D6 marker substrate for the assessment of CYP2D6 inhibition potential [10]. It is an antitussive drug and is used for temporary relief of coughs that are caused by certain infections of the air passages. In addition, dextromethorphan may be co-administered with emvododstat in clinical studies in COVID-19 subjects and potentially in clinical practice in the future. Dextromethorphan is given every 4 to 12 h as needed. As such, this study was designed to evaluate the maximum effect of emvododstat on the pharmacokinetics (PK) of dextromethorphan if there is any.

The study was conducted in accordance with the principles outlined in the 1964 Declaration of Helsinki and its later amendments and the International Conference on Harmonisation Good Clinical Practice. The Independent Ethics Committee (Evaluation of Ethics in Biomedical Research) reviewed and approved the clinical study protocol, any clinical study protocol amendments, subject information sheets, written informed consent forms, and other relevant documentation. Before participating in the study, each subject was apprised of the nature and purpose of the study, and written informed consent was obtained.

Subjects were healthy, non-smokers with body mass index (BMI) between 18 and 30 kg/m2 and weight ranging from 50 to 110 kg. Subjects were instructed to use only approved contraceptives; other prescribed medicines were stopped 30 days prior to the first dose. Methylxanthine-containing beverages, grapefruit juice, and herbal supplements were stopped 48 h, 72 h, and 14 days prior to dosing, respectively. All subjects were genotyped for CYP2D6 status, and poor metabolizers (PMs) were excluded. Subjects were in good health, signed an informed consent form, and did not participate in other trials within 30 days of dosing.

PTC299 (4-chlorophenyl 6-chloro-1-[4-methoxyphenyl]-1,3, 4,9-tetrahydro-2H-pyrido[3,4-b]indole-2-carboxylate) is a proprietary tablet provided by PTC Therapeutics (South Plainfield, NJ, USA). Commercially available dextromethorphan hydrobromide monohydrate 40.2 mg equivalent to 30 mg dextromethorphan (DARO Retard capsules) was purchased by PRA Health Sciences (ICON, Netherlands).

On day 1, 18 healthy subjects (ages ≥ 18 and ≤ 65 years) received a single oral dose of 30 mg (capsule) dextromethorphan followed by a 4-day washout period. On day 5, subjects received a single oral dose of 250 mg emvododstat with food. Two hours later, subjects were administered a single oral dose of 30 mg modified-release dextromethorphan.

Blood samples (3 mL) for dextromethorphan and dextrorphan assessments were collected in K2EDTA vacutainers on day 1 (dextromethorphan alone) and on day 5 (dextromethorphan in combination with emvododstat) at the following timepoints: 0- (predose), 0.5-, 1-, 2-, 3-, 4-, 5-, 6-, 8-, 12-, 24-, and 48-h post dextromethorphan dose. Within 120 min of collection of blood samples, plasma was harvested by centrifuging blood samples at 2000 ± 20 g for 10 min at 4 °C. Upon harvesting, plasma was transferred to 2-mL polypropylene tubes and immediately stored at – 20 ± 5 °C until analysis.

The safety and tolerability of dextromethorphan alone and with emvododstat were monitored.

Concentrations of dextromethorphan and dextrorphan in human plasma samples were determined using high-performance liquid chromatography with tandem mass spectrophotometric (LC-MS/MS) detection. The bioanalytical method was validated, and its performance during bioanalysis was within the limits specified in current regulatory guidance over a calibration range of 50 to 50,000 pg/mL. Validation included testing such as linearity, sensitivity, specificity, selectivity, accuracy and precision, dilution effects, recovery, ruggedness, and stability.

Briefly, dextromethorphan and dextrorphan were extracted from 0.2 mL of human K2EDTA plasma using an automated liquid–liquid extraction followed by detection using LC-MS/MS. Stable isotope labeled internal standard (dextromethorphan-d3 and dextrorphan-d3) was used to track the analytes potential variability. To 0.2 mL of plasma, the internal standard (IS) working solution diluted in an ammonium formate/ammonium hydroxide buffer solution was added. The analytes and ISs were extracted using methyl-tert-butyl-ether (MTBE). A portion of the upper layer was transferred to a 96 deep well collection plate using an automated liquid handling system. After evaporation to dryness, the dry residue was reconstituted with the mobile phase and injected on a Sciex API 4000. The chromatographic separation was performed in isocratic mode using BetaSil CN column (dimensions 50 × 4.6 mm, 5 μm) and methanol/Milli-Q type water with ammonium formate and formic acid as mobile phase. The mass spectrometer detection was performed in positive ion mode with multiple reaction monitoring (MRM) transitions of 272.2 → 215.3, 275.2 → 215.3, 258.2 → 201.2, and 261.2 → 157.2 for dextromethorphan, dextromethorphan-d3, dextrorphan, and dextrorphan-d3, respectively.

Dextromethorphan and dextrorphan concentrations in study plasma samples were determined using duplicate plasma calibration curves of dextromethorphan and dextrorphan in each analytical batch. Additionally, duplicate quality control plasma samples spiked with dextromethorphan and dextrorphan at four concentration levels (low, 150 pg/mL; intermediate, 2000 pg/mL; medium, 25,000 pg/mL; and high, 375,000 pg/mL) were included in each analytical batch to assess the assay performance of the bioanalytical method. The accuracy (%bias) of QC samples in all analytical batches was within ± 4.6%, and precision (%coefficient of variation) was less than 6.2% for dextromethorphan and dextrorphan. The method performance was further demonstrated with 100% passing rate during the conduct of incurred sample reproducibility (ISR) assessment.

Non-compartmental pharmacokinetics were performed using Phoenix® WinNonlin (Version 8.1, Certara, Princeton, NJ, USA). Primary parameters consisted of AUC0-last, AUC0-inf, and Cmax. Secondary parameters included T1/2, Tmax, and CL/F (drug clearance). Bioavailability assessments were calculated using SAS software (Version 9.4, SAS, Cary, NC, USA) in accordance with current regulatory guidelines [11, 12]. A mixed linear model was fitted on log-transformed AUC0-inf, AUC0-t, and Cmax for both parent and metabolite. The model included terms for treatment (combined emvododstat + dextromethorphan as test treatment versus dextromethorphan as reference treatment) and subject as a random factor. LSM (geometric least squares mean) for the treatment ratio and 90% confidence intervals (90% CI) were estimated from the model.