Development of radioiodinated lipophilic cationic compounds for myocardial imaging

Reduced P-glycoprotein recognition of a radioiodine-labeled phosphonium cation by stilbenylation for mitochondrial membrane potential based-imaging

The accumulation of radiolabeled phosphonium cations in cells is dependent on the mitochondrial membrane potential (MMP). However, the efflux of these cations from tumor cells via P-glycoprotein (P-gp) limits their clinical application as MMP-based imaging tracers. In the present study, we designed (E)-diethyl-4-[¹²⁵I]iodobenzyl-4-stilbenylphosphonium ([¹²⁵I]IDESP), which contains a stilbenyl substituent, as a P-gp inhibitor to reduce P-gp recognition, and evaluated its biological properties in comparison with 4-[¹²⁵I]iodobenzyl dipropylphenylphosphonium ([¹²⁵I]IDPP). The in vitro cellular uptake ratio of [¹²⁵I]IDESP in P-gp expressing K562/Vin cells to the parent (P-gp negative) K562 cells was significantly higher than that of [¹²⁵I]IDPP. The efflux rate of [¹²⁵I]IDESP was not significantly different between K562 and K562/Vin, while [¹²⁵I]IDPP was rapidly effluxed from K562/Vin compared with K562, and the efflux of [¹²⁵I]IDPP from K562/Vin was inhibited by the P-gp inhibitor, cyclosporine A. The cellular uptake of [¹²⁵I]IDESP was well correlated with the MMP levels. These results suggested that [¹²⁵I]IDESP was accumulated in cells depending on the MMP levels, without being effluxed via P-gp, while [¹²⁵I]IDPP was rapidly effluxed from the cells via P-gp. Despite having suitable in vitro properties for MMP-based imaging, [¹²⁵I]IDESP showed rapid blood clearance and lower tumor accumulation than [¹²⁵I]IDPP. Improvement in the normal tissue distribution of [¹²⁵I]IDESP is required to develop an agent for use in in vivo MMP-based tumor imaging.

Preparation and biological evaluation of radioiodine-labeled triphenylphosphine derivatives as mitochondrial targeting probes

The positive-charged lipophilic triphenylphosphonium cations (TPPs+ ) have been served as mitochondrial targeting vehicles for the delivery of various probes. In this study, we developed a new method for the preparation of radioiodine-labeled TPPs+ . Four 125 I-labeled TPPs+ , [125 I] 9-[125 I] 12, were prepared from the corresponding triphenylphosphine phenylborate precursors of B 5-B 8 via an optimized copper-catalyzed one-step procedure in high radiochemical yield (>95%). After radio-HPLC purification, the final products could be obtained with high specific activity. Their physicochemical properties, in vitro cellular uptake, and ex vivo mice biodistribution were investigated. The results suggested the 125 I-labeled TPPs+ were lipophilic and could specifically accumulate in the mitochondrial-rich myocardial cells through the mitochondrial membrane potential.

Synthesis and evaluation of novel radioiodinated anthranilate derivatives for in vivo imaging of vascular endothelial growth factor receptor with single-photon emission computed tomography

OBJECTIVE

Angiogenesis facilitates tumor survival and promotes malignancy. The vascular endothelial growth factor (VEGF)/VEGF receptor (VEGFR) tyrosine kinase (TK) signaling pathway is a key factor mediating angiogenesis, suggesting that this pathway may be a target for diagnosis and therapy. In this study, we aimed to develop small molecule radioiodinated probes applicable for in vivo VEGFR imaging considering the versatility and usefulness of single-photon emission computed tomography (SPECT).

METHODS

We designed and synthesized four radioiodinated anthranilate compounds (6a-d) based on the structure of an anticancer drug targeting VEGFR-TK. The inhibitory potencies of corresponding cold compounds 4a-d and in vitro stability of compounds 6a-d were assessed by cellular proliferation inhibition assays and radio thin-layer chromatography after incubation in neutral solution. In vivo biodistributions were evaluated by determining radioactivity in tissues of interest after intravenous injection of test compounds in tumor-bearing mice. In vitro and in vivo blocking experiments using a selective VEGFR-TK inhibitor and SPECT/computed tomography (CT) imaging were performed in tumor-bearing mice.

RESULTS

The radioiodinated compounds 6a-d were obtained with more than 68.0% radiochemical yield and more than 95% radiochemical purity. Because compounds 4a-d showed high inhibitory potencies and compounds 6c and 6d showed high in vitro stability, 6c ([¹²⁵I]m-NPAM) and 6d ([¹²⁵I]p-NPAM) were further evaluated. Analysis of the in vivo biodistribution revealed a tumor to blood radioactivity ratio of greater than 4 at 24 h after [¹²⁵I]p-NPAM administration. Accumulation of radioactivity in cultured tumor cells and tumor xenografts after [¹²⁵I]p-NPAM administration was significantly blocked by inhibitor pretreatment. Tumors were clearly imaged at 24 h after [¹²⁵I]p-NPAM injection with SPECT/CT in comparison to that in inhibitor-pretreated tumor-bearing mice.

CONCLUSION

[¹²⁵I]p-NPAM may have potential applications as a lead compound for future development of a clinically usable VEGFR imaging probe for SPECT.

Radiosynthesis and preclinical evaluation of [18F] 4-(2-fluoroethoxy)-2H-chromen-2-one as a novel myocardial perfusion imaging agent

Recently we developed [18F] 4-(2-fluoroethoxy)-2H-chromen-2-one as a novel 18F myocardial perfusion imaging radiotracer. It was synthesized in good radiochemical yield (>90%). The total time from radiosynthesis to its purification was less than 40 min, with excellent radiochemical purity (≥99%). It showed good stability over a period of 5 h at room temperature. The partition coefficient (log P) of radiotracer was found to be 2.70, suggesting the lipophilic nature of radiotracer. Ex vivo biodistribution study of radiotracer in normal Wistar rats for 30 min post-injection, demonstrated a good heart uptake (>1.3% ID/g) and favorable pharmacokinetics. Additionally, the radiotracer showed significant excretion (>11% ID) by liver, which is indicative of its rapid clearance. Further, in vivo biodistribution study of radiotracer in New Zealand White rabbit provided the clear PET/CT images of cardiomyocytes and myocardial perfusion. All these experimental findings suggest that [18F] 4-(2-fluoroethoxy)-2H-chromen-2-one could be used as a potential hit for myocardial perfusion imaging.