PMC7446343_01
Clinical activity of LOXO-292 in a patient with RET fusion-positive non-small-cell lung cancer after acquired resistance to vandetanib. (A) Various treatments the patient received for metastatic, KIF5B-RET fusion-positive non-small-cell lung cancer are shown, together with the levels of carcinoembryonic antigen (CEA; red diamonds) and the sum of target lesions by Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 (blue squares) over time during treatment. The RET molecular profiles of tumor specimens obtained before and during vandetanib treatment are shown in boxes. (B) Pharmacokinetic analysis at steady state after treatment with LOXO-292 at a dose of 160 mg twice per day for 8 days. The estimated levels of RET target inhibition at the 50% inhibitory concentration (IC50) and 90% inhibitory concentration (IC90) for KIF5B-RET and KIF5B-RET-V804L were modeled using actual patient human pharmacokinetic parameters (Data Supplement). (C) Computed tomographic images of the patient's metastatic lung and liver disease before and at the indicated times after initiating treatment with vandetanib (left three pairs of panels) or LOXO-292 (right two pairs of panels). AMR, ambirubicin; DTX, docetaxel; PD, progressive disease; PEM, pemetrexed; PR, partial response; S1, tegafur plus gimeracil plus oteracil.
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She received alectinib and additional chemotherapy and then vandetanib (Fig 1A). Tumors (RECIST) 1.1 with vandetanib treatment, with improvement in multiple lung and pleural-based lesions. She progressed after 13 months of treatment (Figs 1A and 1C). Targeted RET gene sequencing of a biopsy specimen from a progressing right lung lesion identified a RET valine-to-leucine (V804L) gatekeeper mutation absent from a pretreatment tumor sample (Fig 1A, Data Supplement). Real-time PK analysis revealed significant (more than 90% inhibitory concentration [IC90]) calculated KIF5B-RET and KIF5B-RET-V804L target coverage at steady state (Fig 1B). Consistent with this, she experienced a rapid clinical and biochemical response to LOXO-292, with decreased shortness of breath and reduction in serum carcinoembryonic antigen (Fig 1A). Repeat imaging after 8 weeks of treatment demonstrated a 48% decrease in measurable tumor burden by RECIST 1.1 that was maintained after 16 weeks, indicating a confirmed PR (Figs 1A and 1C).
PMC7446343_02
Clinical activity of LOXO-292 in a patient with RET-mutant medullary thyroid cancer with a germline V804M gatekeeper mutation. (A) The treatments the patient received for metastatic, RET V804M-mutant medullary thyroid cancers are shown. (B) Pharmacokinetic analysis at steady state after 8 days of treatment with LOXO-292 at a dose of 80 mg, 120 mg, and 160 mg twice a day. The estimated levels of RET target inhibition at the 50% inhibitory concentration (IC50) and 90% inhibitory concentration (IC90) for wild-type (WT) RET and RET V804M were modeled as in Fig 1B. (C) Levels of calcitonin (green circles) and carcinoembryonic antigen (CEA; red diamonds) over time during treatment. (D) Computed tomographic images of the patient's metastatic liver disease before and after 8 weeks of treatment with LOXO-292. The red arrows indicate diffuse infiltration of the liver by tumor. Lenvat, lenvatinib; RT, radiation treatment; Vande, vandetanib.
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A 50-year-old man with hereditary RET V804M multiple endocrine neoplasia 2A and MTC developed primary disease progression despite previous treatment with three anti-RET MKIs (Fig 2A). PK analysis indicated more than 50% inhibitory concentration calculated RET V804M target coverage at this dose (Fig 2B). Repeat imaging after 8 weeks of treatment indicated a complete response by RECIST 1.1, which was confirmed after 12 weeks of treatment, with complete resolution of target mediastinal lymph node lesions and nontarget liver metastases and normalization of serum tumor markers (Figs 2C and 2D). The patient's dose was increased to 160 mg twice a day as allowed by the protocol, with dose-proportional increases in exposure and continuous more than IC90 RET V804M target coverage (Fig 2B), continued complete response, and continued normalization of tumor markers (Fig 2C).
RET inhibitor binding to RET resistance mutations. (A) Structural models showing steric interactions (circled red) between vandetanib (Vande) and gatekeeper (V804L and V804M) resistance mutations in RET. (B) Similar model indicating that LOXO-292 can accommodate V804L and V804M without significant steric hindrance. (C) The 50% inhibitory concentration (IC50) values for Vande, cabozantinib (Cabo), and LOXO-292 in (left) purified kinase assays at 1mM ATP and (right) cell-based assays of RET autophosphorylation are shown as mean +- SEM of 5 to 145 (enzyme) and 2 to 55 (cell) replicates.
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V804L and V804M substitutions in the gatekeeper residue of RET are predicted to introduce steric clashes between the leucine and methionine side chains and the 4-bromo-2-fluorophenyl group of the anti-RET MKI vandetanib (Fig 3A). In contrast to MKIs, LOXO-292 is predicted to better accommodate the bulky leucine and methionine side chains in the gatekeeper residues without any steric interactions (Fig 3B). To determine the impact of RET kinase domain resistance mutations on inhibitor activity, LOXO-292, vandetanib, and cabozantinib were tested in vitro against the purified wild-type and mutant RET kinase domains (Fig 3C). These findings were confirmed in cell-based assays of RET autophosphorylation (Fig 3C).