12.1 Mechanism of Action
Larotrectinib is an inhibitor of the tropomyosin receptor kinases (TRK), TRKA, TRKB, and TRKC. In a broad panel of purified enzyme assays, larotrectinib inhibited TRKA, TRKB, and TRKC with IC50 values between 5-11 nM. One other kinase TNK2 was inhibited at approximately 100-fold higher concentration. TRKA, B, and C are encoded by the genes NTRK1, NTRK2, and NTRK3. Chromosomal rearrangements involving in-frame fusions of these genes with various partners can result in constitutively-activated chimeric TRK fusion proteins that can act as an oncogenic driver, promoting cell proliferation and survival in tumor cell lines.
In in vitro and in vivo tumor models, larotrectinib demonstrated anti-tumor activity in cells with constitutive activation of TRK proteins resulting from gene fusions, deletion of a protein regulatory domain, or in cells with TRK protein overexpression. Larotrectinib had minimal activity in cell lines with point mutations in the TRKA kinase domain, including the clinically identified acquired resistance mutation, G595R. Point mutations in the TRKC kinase domain with clinically identified acquired resistance to larotrectinib include G623R, G696A, and F617L.
At a dose 9-fold higher than the recommended adult dose, VITRAKVI does not prolong QTc intervals to any clinically relevant extent.
The pharmacokinetics of larotrectinib were studied in healthy subjects and adult and pediatric patients with locally advanced or metastatic solid tumors. In healthy subjects who received a single dose of VITRAKVI capsules, systemic exposure (Cmax and AUC) of larotrectinib was dose proportional over the dose range of 100 mg to 400 mg (1 to 4 times the recommended adult dose) and slightly greater than proportional at doses of 600 mg to 900 mg (6 to 9 times the recommended adult dose). In adult patients who received VITRAKVI capsules 100 mg twice daily in Study LOXO-TRK-14001, peak plasma levels (Cmax) of larotrectinib were achieved at approximately 1 hour after dosing and steady-state was reached within 3 days. Mean steady-state larotrectinib [coefficient of variation (CV%)] for Cmax was 788 (81%) ng/mL and AUC0-24hr was 4351 (97%) ng*h/mL.
The mean absolute bioavailability of VITRAKVI capsules was 34% (range: 32% to 37%). In healthy subjects, the AUC of VITRAKVI oral solution was similar to that of the capsules and the Cmax was 36% higher with the oral solution.
Effect of Food
The AUC of larotrectinib was similar and the Cmax was reduced by 35% after oral administration of a single 100 mg capsule of VITRAKVI to healthy subjects taken with a high-fat meal (approximately 900 calories, 58 grams carbohydrate, 56 grams fat and 43 grams protein) compared to the Cmax and AUC in the fasted state.
The mean (CV%) volume of distribution (Vss) of larotrectinib is 48 (38%) L following intravenous administration of larotrectinib in healthy subjects.
Larotrectinib is 70% bound to human plasma proteins in vitro and binding is independent of drug concentrations. The blood-to-plasma concentration ratio is 0.9.
The mean (CV%) clearance (CL/F) of larotrectinib is 98 (44%) L/h and the half-life is 2.9 hours following oral administration of VITRAKVI in healthy subjects.
Larotrectinib is metabolized predominantly by CYP3A4. Following oral administration of a single [14C] radiolabeled 100 mg dose of larotrectinib to healthy subjects, unchanged larotrectinib constituted 19% and an O-linked glucuronide constituted 26% of the major circulating radioactive drug components in plasma.
Following oral administration of a single [14C] radiolabeled 100 mg dose of larotrectinib to healthy subjects, 58% (5% unchanged) of the administered radioactivity was recovered in feces and 39% (20% unchanged) was recovered in urine.
Age (range: 28 days to 82 years), sex, and body weight (range: 3.8 kg to 179 kg) had no clinically meaningful effect on the pharmacokinetics of larotrectinib.
In pediatric patients, the larotrectinib geometric mean (%CV) AUC0-24hr by age subgroup was: 3348 (66%) ng*h/mL in patients 1 month to < 2 years (n = 9), 4135 (36%) ng*h/mL in patients 2 to < 12 years (n = 15), and 3108 (69%) ng*h/mL and in patients 12 to < 18 years (n = 9).
Patients with Renal Impairment
Following oral administration of a single 100 mg dose of VITRAKVI capsules in subjects with end-stage renal disease (e.g., subjects who required dialysis), the AUC0-INF of larotrectinib increased 1.5-fold and Cmax increased 1.3-fold as compared to that in subjects with normal renal function (creatinine clearance ≥ 90 mL/min as estimated by Cockcroft-Gault). The pharmacokinetics of VITRAKVI in patients with moderate to severe renal impairment (creatinine clearance ≤ 60 mL/min) have not been studied.
Patients with Hepatic Impairment
Following oral administration of a single 100 mg dose of VITRAKVI capsules, the AUC0-INF of larotrectinib increased 1.3-fold in subjects with mild hepatic impairment (Child-Pugh A), 2-fold in subjects with moderate hepatic impairment (Child-Pugh B) and 3.2-fold in subjects with severe hepatic impairment (Child-Pugh C) as compared to that in subjects with normal hepatic function. The Cmax was similar in subjects with mild and moderate hepatic impairment and the Cmax of larotrectinib increased 1.5-fold in subjects with severe hepatic impairment as compared to that in subjects with normal hepatic function [see Dosage and Administration (2.6), Use in Specific Populations (8.6)].
Drug Interaction Studies
Effect of Strong CYP3A Inhibitors: Coadministration of a single 100 mg dose of VITRAKVI capsules with a strong CYP3A inhibitor (itraconazole) increased the AUC0-INF of larotrectinib by 4.3-fold and the Cmax by 2.8-fold as compared to VITRAKVI administered alone [see Dosage and Administration (2.4), Drug Interactions (7.1)]. The effects of CYP3A moderate and weak inhibitors on the pharmacokinetics of larotrectinib have not been studied.
Effect of Strong CYP3A Inducers: Coadministration of a single 100 mg dose of VITRAKVI capsules with a strong CYP3A inducer (rifampin) decreased the AUC0-INF of larotrectinib by 81% and of Cmax by 71% as compared to VITRAKVI administered alone [see Dosage and Administration (2.5), Drug Interactions (7.1)]. The effects of CYP3A weak and moderate inducers on the pharmacokinetics of larotrectinib have not been studied.
Effect of Strong P-glycoprotein (P-gp) Inhibitors: Coadministration of a single 100 mg dose of VITRAKVI capsules with a P-gp inhibitor (rifampin) increased the AUC0-INF of larotrectinib by 1.7-fold and the Cmax by 1.8-fold as compared to VITRAKVI administered alone.
Effect of Larotrectinib on CYP3A4 Substrates: Coadministration of VITRAKVI capsules 100 mg twice daily with a sensitive CYP3A4 substrate (midazolam) increased both the AUC0-INF and Cmax of midazolam by 1.7-fold as compared to midazolam administered alone. The AUC0-INF and Cmax of 1-hydroxymidazolam, the main metabolite of midazolam, were both increased 1.4-fold as compared to when midazolam was administered alone [see Drug Interactions (7.2)].
In Vitro Studies
Effect of Transporter on Larotrectinib: Larotrectinib is a substrate for P-gp and BCRP. Larotrectinib is not a substrate of OAT1, OAT3, OCT1, OCT2, OATP1B1, or OATP1B3.
Effect of Larotrectinib on Transporters: Larotrectinib is not an inhibitor of BCRP, P-gp, OAT1, OAT3, OCT1, OCT2, OATP1B1, OATP1B3, BSEP, MATE1 and MATE2-K at clinically relevant concentrations.
Effect of Larotrectinib on CYP Substrates: Larotrectinib is not an inhibitor or inducer of CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, or CYP2D6 at clinically relevant concentrations.