Fibroblast activation protein (FAP) has received increasing attention as an oncologic target because of its prominent expression in solid tumors but virtual absence from healthy tissues. Most radioligand therapies (RLTs) targeting FAP, however, suffer from inadequate tumor retention or clearance from healthy tissues. Herein we report a FAP-targeted RLT comprising an FAP6 ligand conjugated to DOTA and an albumin binder (4-p-iodophenylbutyric acid, or IP) for enhanced pharmacokinetics. We evaluated the performance of the resulting FAP6-IP-DOTA conjugate in 4 tumor models, 3 of which express FAP only on cancer-associated fibroblasts, that is, analogously to human tumors. Methods: Single-cell RNA-sequencing data were analyzed from 34 human breast, ovarian, colorectal, and lung cancers to quantify FAP-overexpressing cells. FAP6-DOTA conjugates were synthesized with or without an albumin binder (IP) and investigated for binding to human FAP-expressing cells. Accumulation of ¹¹¹In- or ¹⁷⁷Lu-labeled conjugates in KB, HT29, U87MG, and 4T1 murine tumors was also assessed by radioimaging or biodistribution analyses. Radiotherapeutic potency was quantitated by measuring tumor volumes versus time. Results: Approximately 5% of all cells in human tumors overexpressed FAP (cancer-associated fibroblasts comprised ∼77% of this FAP-positive subpopulation, whereas ∼2% were cancer cells). FAP6 conjugates bound to FAP-expressing cells with high affinity (dissociation constant, ∼1 nM). ¹⁷⁷Lu-FAP6-IP-DOTA achieved an 88-fold higher tumor dose than ¹⁷⁷Lu-FAP6-DOTA and improved all tumor-to-healthy-organ ratios. Single doses of ¹⁷⁷Lu-FAP6-IP-DOTA suppressed tumor growth by about 45% in all tested tumor models without causing reproducible toxicities. Conclusion: We conclude that ¹⁷⁷Lu-FAP6-IP-DOTA constitutes a promising candidate for FAP-targeted RLT of solid tumors.

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