After androgen deprivation therapy, a significant number of prostate cancer cases progress with a therapy-resistant neuroendocrine phenotype (NEPC). This represents a challenge for diagnosis and treatment. Based on our previously reported design of theranostic small-molecule prodrug conjugates (T-SMPDCs), herein we report a T-SMPDC tailored for targeted positron emission tomography (PET) imaging and chemotherapy of NEPC. The T-SMPDC is built upon a triazine core (TZ) to present three functionalities: (1) a chelating moiety (DOTA: 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) for PET imaging when labeled with ⁶⁸Ga (t_1/2 = 68 min) or other relevant radiometals; (2) an octreotide (Octr) that targets the somatostatin receptor 2 (SSTR2), which is overexpressed in the innervated tumor microenvironment (TME); and (3) fingolimod, FTY720—an antagonist of sphingosine kinase 1 that is an intracellular enzyme upregulated in NEPC. Polyethylene glycol (PEG) chains were incorporated via conventional conjugation methods or a click chemistry reaction forming a 1,4-disubstituted 1,2,3-triazole (Trz) linkage for the optimization of in vivo kinetics as necessary. The T-SMPDC, DOTA-PEG₃-TZ(PEG₄-Octr)-PEG₂-Trz-PEG₃-Val-Cit-pABOC-FTY720 (PEG_n: PEG with n repeating ethyleneoxy units (n = 2, 3, or 4); Val: valine; Cit: citrulline; pABOC: p-amino-benzyloxycarbonyl), showed selective SSTR2 binding and mediated internalization of the molecule in SSTR2 ^high cells. Release of FTY720 was observed when the T-SMPDC was exposed to cathepsin B, and the released FTY720 exerted cytotoxicity in cells. In vivo PET imaging showed significantly higher accumulation (2.1 ± 0.3 %ID/g; p = 0.02) of [⁶⁸Ga]Ga-DOTA-PEG₃-TZ(PEG₄-Octr)-PEG₂-Trz-PEG₃-Val-Cit-pABOC-FTY720 in SSTR2^high prostate cancer xenografts than in the SSTR2^low xenografts (1.5 ± 0.4 %ID/g) at 13 min post-injection (p.i.) with a rapid excretion through the kidneys. Taken together, these proof-of-concept results validate the design concept of the T-SMPDC, which may hold a great potential for targeted diagnosis and therapy of NEPC.

Background: Fibroblast activation protein (FAP)-targeted theranostic radiopharmaceuticals have shown desired tumor-to-background organ selectivity due to the ubiquitous presence of FAP within the tumor microenvironment. However, suboptimal tumor retention and fast clearance have hindered their use to deliver effective cancer therapies. With well-documented FAP-targeting moieties and linkers appending them to optimal chelators, the development of copper radiopharmaceuticals has attracted considerable interest, given the fact that an ideal theranostic pair of copper radionuclides (⁶⁴Cu: t_1/2 = 12.7 h; 17.4% β⁺; E_β⁺_max = 653 keV and ⁶⁷Cu: t_1/2 = 2.58 d; 100% β⁻; E_β⁻_max = 562 keV) are available. Herein, we report our design, synthesis, and comparative evaluation of monovalent and divalent FAP-targeted theranostic conjugates constructed from our previously reported bifunctional chelator scaffold (BFS) based on 1,4,8,11-tetraaza-bicyclo [6.6.2]hexadecane-4,11-diacetic acid (CB-TE2A), which forms the most stable complex with Cu(II). Methods: After synthesis and characterization, the monovalent and divalent conjugates were radiolabeled with ⁶⁴Cu for in vitro cell assays, followed by in vivo positron emission tomography (PET) imaging evaluation in relevant mouse models. Results: Both ⁶⁴Cu-labeled conjugates showed high in vitro stability and anticipated FAP-mediated cell binding and internalization. The divalent one showed significantly higher FAP-specific tumor uptake than its monovalent counterpart. Conclusions: Our results demonstrate that the BFS-based multivalent approach can be practically used to generate FAP-targeted radiotheranostic agents for effective cancer diagnosis and treatment.