(G-244) Same-day Dual-Tracer Imaging of Receptors Targeted by a Bispecific Antibody: Insights from Proof of Concept Studies

Amivantamab (AMI) is a bispecific antibody (bsAb) that binds to the epidermal growth factor receptor (EGFR) and the cytoplasmic mesenchymal-epithelial transition factor (cMET) receptors and inhibits tumor growth in non-small cell lung cancer (NSCLC). AMI was FDA-approved in 2021 for the treatment of patients with metastatic NSCLC who harbor exon 20 mutations in EGFR. AMI is a promising therapy but not all patients respond to it. Current techniques to select candidates likely to respond to bsAb treatment are limited to immunohistochemical and mutation analysis of biopsies. These methods are highly invasive and binding sites for bsAb in biopsies may not be the same as for metastatic lesions in the body. However, PET imaging is noninvasive and may serve as an equal or superior means of patient selection. First, we must overcome biological and technological challenges. Namely, EGFR- and cMET- expression are both expected to change after AMI treatment, but PET cannot discriminate between radionuclides if two tracers are injected simultaneously. In this proof of concept study, we designed a novel imaging paradigm to image both receptors on the same day by sequential administration of a long-lived tracer [⁸⁹Zr]Zr-DFO-anti-cMET (⁸⁹Zr t_1/2 = 78.4 hr) that binds to cMET and a short-lived tracer [¹¹C]erlotinib (¹¹C t_1/2 = 20.4 min) that binds to EGFR mutants. Here, we evaluate changes in receptor expression in vivo over the course of treatment while overcoming limitations of the standard diagnostic techniques.

We used HCC827 NSCLC xenografts in nude mice as a model.  [⁸⁹Zr]Zr-DFO-anti-cMET  (41. 3 ± 3.9 µCi) was injected via tail vein (n=2), followed by acquisition of static 10-min PET scans (Focus 220) at 2 hr, 1, 2, and 3 days post injection. The Focus exports images that have been decay-corrected for a selected isotope. On day 3, to subtract the contribution of ⁸⁹Zr activity from the subsequent [¹¹C]erlotinib scan, a 5-min static (“pre-injection”) scan was done using ¹¹C acquisition parameters. Following this scan, 130 ± 28 µCi of [¹¹C]erlotinib was injected and a dynamic scan was acquired for 120 min. Because the images from day 3 contain effects of two tracers with different isotopes, the decay-correction was removed from images of [¹¹C]erlotinib on day 3. The ⁸⁹Zr activity contribution was subtracted from the ¹¹C scan based on the activity in the 5-min “pre-injection scan.” Finally, the corrected [¹¹C]erlotinib scan was decay corrected. Starting on day 5, animals were treated with AMI (10 mg/kg 2x per week) for 25 days. Following a 7-day washout period, dual-tracer imaging was repeated. Tumor, reference tissue and heart ROIs were segmented, and time activity curves were generated from each animal. Binding potential (BP) for the ⁸⁹Zr-anti-cMET was calculated by Logan Plot; and the tumor uptake rate constant (Ki) was calculated for [¹¹C]erlotinib using Patlak Plot. Analyses were performed before and after treatment with AMI.

In both animals, standard uptake values (SUV) for each tumor decreased (see figure). BP of [⁸⁹Zr]Zr-DFO-anti-cMET was reduced by >66% in animal-1(pre- = 14.1, and post-therapy = 4.7, respectively)  and >42% in animal-2 (7.5, and 4.3, respectively).  Ki of 11C-erlotinib was reduced by >30% in animal-1 (pre- = 0.0129, and post-therapy = 0.0088 min-1, respectively) and >42% in animal-2 (0.0129, and 0.0122, respectively).

Preliminary analysis (n=2) suggests that same-day dual-tracer PET studies are feasible and that after proper correction for the presence of multiple isotopes, the images offer quantitative measures of receptors targeted by bsAb, which could be used to determine response to treatment. Further imaging with full cohorts of tumor-bearing and control animals will be necessary to confirm a response of tumors to AMI therapy with dual-tracer PET.