Cyclopentadienyl-Based Amino Acids (Cp-aa) As Phenylalanine Analogues for Tumor Targeting: Syntheses and Biological Properties of [(Cp-aa)M(CO)3](M = Mn, Re, 99mTc)

Due to an enhanced demand for amino acids, the l-type amino acid transporter 1 (LAT1) is overexpressed in many tumor cell lines. LAT1 represents therefore an attractive target for cancer therapy and diagnosis. On the basis of our reported aqueous synthesis of [(Cp-R)99mTc­(CO)3]-type complexes,− we describe the preparation of unnatural amino acid analogues [(Cp-CH2CH­(NH2)­COOH)­Mn­(CO)3] and [(Cp-CH­(NH2)­COOH)­M­(CO)3] (M = Mn, Re, 99mTc). Starting from fully protected HC5H5-aa (aa = amino acid), [(Cp-aa)99mTc­(CO)3] complexes are accessible in quantitative yields and in a one-step synthesis from [99mTcO4]−. The rhenium and manganese analogues were prepared and structurally characterized to confirm the authenticity of the 99mTc complex. The inhibition constant of natural phenylalanine (phe) for LAT1 is in the range 70 ± 10 μM. The Ki value of [(Cp-CH­(NH2)­COOH)­Mn­(CO)3] (1a) is 53 ± 11 μM, whereas Ki for the “true” phe analogue [(Cp-CH2CH­(NH2)­COOH)­Mn­(CO)3] (2) was surprisingly high at 277 ± 37 μM. Complex 1a caused efflux when exposed to cells, underlining its active transport by LAT1 into the cell. 99mTc analogues of small biological lead structures such as amino acids are generally not recognized anymore by their targets, in particular by trans-membrane transporters. The bioorganometallic analogues presented here are, however, actively transported and corroborate the importance of organometallic complexes as mimics of organic lead structures in life sciences.

鉴于氨基酸需求攀升，L型氨基酸转运体1（L-type amino acid transporter 1, LAT1）在多种肿瘤细胞系中呈过表达态势，因此成为癌症治疗与诊断的极具潜力的靶点。基于我们此前报道的[(Cp-R)99mTc(CO)3]-型配合物的水相合成策略，本文报道了非天然氨基酸类似物[(Cp-CH2CH(NH2)COOH)Mn(CO)3]与[(Cp-CH(NH2)COOH)M(CO)3]（M = Mn, Re, 99mTc）的制备方法。以完全保护的HC5H5-aa（aa = 氨基酸）为起始原料，[(Cp-aa)99mTc(CO)3]配合物可由高锝酸根（[99mTcO4]-）经一步反应以定量收率合成得到。我们制备了铼与锰的对应类似物，并对其进行结构表征，以验证该锝-99m（99mTc）配合物的结构真实性。天然苯丙氨酸（phe）对LAT1的抑制常数为70 ± 10 μM。[(Cp-CH(NH2)COOH)Mn(CO)3]（1a）的Ki值为53 ± 11 μM，而作为"真正的"苯丙氨酸类似物的[(Cp-CH2CH(NH2)COOH)Mn(CO)3]（2）的Ki值却高达277 ± 37 μM，这一结果颇为出人意料。配合物1a在与细胞共孵育时可引发外排效应，证实其可通过LAT1介导的主动转运进入细胞。通常而言，氨基酸这类小型生物先导结构的锝-99m（99mTc）类似物往往无法再被其靶点识别，尤其是跨膜转运蛋白。但本文所报道的生物有机金属配合物类似物却可被主动转运，这进一步佐证了有机金属配合物作为生命科学领域有机先导结构模拟物的重要价值。