571P Preclinical evaluation of FAP-2286, a peptide-targeted radionuclide therapy (PTRT) to fibroblast activation protein alpha (FAP)

FAP is a membrane bound protease that has limited expression in normal adult tissues but is highly expressed on cancer associated fibroblasts (CAFs), which are abundant in most tumours. FAP-2286 is a conjugate of a FAP-binding peptide coupled to a radionuclide chelator (DOTA) for use as a potential PTRT for imaging and therapeutic applications. The potency, selectivity, and efficacy of FAP-2286 were evaluated in vitro and in vivo. Immunohistochemistry was performed using FFPE tissue specimens. FAP-2286 and its metal complexes were evaluated in in vitro biochemical and cellular assays and in vivo biodistribution, imaging, and efficacy studies. Immunohistochemistry confirmed high levels of FAP expression, predominantly in CAFs, in multiple tumour types including pancreatic, breast, and sarcoma. In contrast, FAP expression in normal tissues was limited. FAP-2286 and its metal complexes had potent affinity to human FAP protein (KD, 0.4–1.4 nM). In cellular assays, FAP-2286 showed effective binding (IC50, 1.65–3.4 nM) to FAP expressing WI-38 fibroblasts. FAP-2286 inhibited FAP protease activity (IC50, 1.6–3.2 nM), whereas limited off target activity was seen against closely related family members. In vivo biodistribution (by SPECT/CT) in mice after IV injection showed rapid uptake of 111In- and 177Lu-labelled FAP-2286 in FAP-positive tumours that was maintained for ≥120 hours, limited background in normal tissues, and a renal clearance mechanism. FAP-2286 labelled with the therapeutic β-particle emitting radionuclide 177Lu, at a specific activity of 30 MBq/nmol (low-dose group) or 60 MBq/nmol (high-dose group), had potent anti-tumour activity in FAP expressing HEK-293 xenografts after a single IV dose: tumour growth inhibition at day 14 posttreatment was 112% and 115%, respectively, with no significant weight loss. In preclinical models, FAP-2286 has a compelling profile for PTRT, including high tumour uptake and retention, and potent efficacy in FAP-positive tumours. IND-enabling nonclinical work is ongoing; clinical studies in a broad spectrum of FAP-positive cancers are planned.