Abstract

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.

Results from nanoScan® PET/CT

• The dynamic PET imaging with [⁶⁸Ga]Ga-DOTA-PEG₃-TZ(PEG₄-Octr)-PEG₂-Trz-PEG₃-Val-Cit-pABOC-FTY720 clearly showed that the radiolabeled agent was capable of differentiating the two phenotypes of PC3 xenografts by their distinct expressions of SSTR2
• We successfully designed, synthesized, and evaluated a T-SMPDC system, DOTA-PEG₃-TZ(PEG₄-Octr)-PEG₂-Trz-PEG₃-Val-Cit-pABOC-FTY720, for NEPC-targeted delivery and controlled intracellular release of an Sphk1 inhibitor to suppress the function of Sphk1

Fig (a) Representative PET/CT images of SCID/NOD mice bearing SSTR2^high PC3_VC and SSTR2^low PC3_sgPTP1B xenografts during a 60-min dynamic PET scan with [⁶⁸Ga]Ga-DOTA-PEG₃-TZ(PEG₄-Octr)-PEG₂-Trz-PEG₃-Val-Cit-pABOC-FTY720 (left) and [⁶⁸Ga]Ga-PSMA-11 (right). Yellow arrows indicate the tumors. (b) Quantitative PET imaging analysis in tissues of interest.

Publication link