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A Pyropheophorbide Analogue Containing a Fused Methoxy Cyclohexenone Ring System Shows Promising Cancer‐Imaging Ability

Herein we report the synthesis, photophysical properties, positron emission tomography (PET) imaging and photodynamic therapy (PDT) efficacy of methyl 3‐(1′‐m‐iodobenzyloxy)ethyl‐3‐devinyl‐verdin 4 (with or without the 124I isotope). The PET imaging ability and ex vivo biodistribution of [124I]4 were compared with the well‐studied methyl [3‐(1241′‐m‐iodobenzyloxy)ethyl]‐3‐devinyl‐pyropheophorbide‐a methyl ester (PET‐ONCO or [124I]2) and [18F]fluorodeoxyglucose ([18F]FDG) in BALB/c mice bearing colon‐26 tumors. Whole‐body PET images of [124I]4 containing a fused methoxy cyclohexenone ring system showed excellent tumor contrast with time (72>48>24 h post‐injection). Ex vivo biodistribution results indicate that relative to the current clinical standard [18F]FDG and [124I]2 in 2 % ethanol formulation, [124I]4, at the same radioactive dose (25 μCi per mouse), showed higher tumor uptake at 24 h post‐injection and longer tumor retention. In biological environments, compound 4 showed lower fluorescence and lower singlet oxygen yield than 2, which is possibly due to higher aggregation caused by the presence of a fused cyclohexenone ring system, resulting in limited in vitro/in vivo PDT efficacy. Therefore, the chlorophyll‐a analogue [124I]4 provides easy access to a novel PET imaging agent (with no skin phototoxicity) to image cancer types—brain, renal carcinomas, pancreas—in which [18F]FDG shows limitations.