García Garayoa Elisa, Rüegg Dominique, Bläuenstein Peter, Zwimpfer Martin, Khan Irfan Ullah, Maes Veronique, Blanc Alain, Beck-Sickinger Annette G, Tourwé Dirk A, Schubiger P August
Paul Scherrer Institute, Centre for Radiopharmaceutical Science, CH-5232 Villigen PSI, Switzerland.
Nucl Med Biol. 2007 Jan;34(1):17-28. doi: 10.1016/j.nucmedbio.2006.10.004. Epub 2006 Nov 28.
Bombesin, a neuropeptide with potential for breast and prostate tumor targeting, is rapidly metabolized in vivo, and as a result, uptake in tumor xenografts in mice is poor. An improvement can be expected from the introduction of nonnatural amino acids and spacers. Leu13 was replaced by cyclohexylalanine and Met14 by norleucine. Two spacers, -betaAla-betaAla- and 3,6-dioxa-8-aminooctanoic acid, were inserted between the receptor-binding amino acid sequence (7-14) of bombesin (BBS) and the retroN(alpha)-carboxymethyl histidine chelator used for labeling with the 99mTc3 core and the rhenium (Re) congener.
The biological characterization of the new compounds was performed both in vitro on prostate carcinoma PC-3 cells (binding affinity, internalization/externalization) and in vivo (biodistribution in nude mice with tumor xenografts). The stability was also investigated in human plasma. The Re analogues were prepared for chemical characterization.
The nonnatural amino acids led to markedly slower degradation in human plasma and PC-3 cell cultures. The receptor affinity of the new technetium 99m ([99mTc])-labeled BBS analogues was similar to the unmodified compound with Kd<1 nM. Uptake in the pancreas and in PC-3 tumor xenografts in nude mice was blocked by unlabeled BBS. The best target-to-nontarget uptake ratio was clearly due to the presence of the more polar spacer, -betaAla-betaAla-.
The different spacers did not have a significant effect on stability or receptor affinity but had a clear influence on the uptake in healthy organs and tumors. Uptake in the kidneys was lower than in the liver, which is likely to be due to the lipophilicity of the compounds. A specific, high uptake was also observed in the gastrin-releasing peptide receptor-rich pancreas. Thus, with the introduction of spacers the in vivo properties of the compounds can be improved while leaving the affinity unaffected.
蛙皮素是一种具有靶向乳腺和前列腺肿瘤潜力的神经肽，在体内会迅速代谢，因此在小鼠肿瘤异种移植中的摄取较差。引入非天然氨基酸和间隔基团有望改善这种情况。亮氨酸13被环己基丙氨酸取代，蛋氨酸14被正亮氨酸取代。在蛙皮素（BBS）的受体结合氨基酸序列（7 - 14）与用于用99mTc3核心和铼（Re）类似物标记的retroN(α)-羧甲基组氨酸螯合剂之间插入了两个间隔基团，即-βAla-βAla-和3,6 - 二氧杂 - 8 - 氨基辛酸。
在体外对前列腺癌PC - 3细胞进行新化合物的生物学特性研究（结合亲和力、内化/外化），并在体内（对带有肿瘤异种移植的裸鼠进行生物分布研究）。还研究了其在人血浆中的稳定性。制备了铼类似物用于化学表征。
非天然氨基酸导致在人血浆和PC - 3细胞培养物中的降解明显减慢。新的99m锝（[99mTc]）标记的BBS类似物的受体亲和力与未修饰的化合物相似，解离常数（Kd）<1 nM。未标记的BBS可阻断裸鼠胰腺和PC - 3肿瘤异种移植中的摄取。最佳的靶标与非靶标摄取比显然归因于极性更强的间隔基团-βAla-βAla-的存在。
不同的间隔基团对稳定性或受体亲和力没有显著影响，但对健康器官和肿瘤中的摄取有明显影响。肾脏中的摄取低于肝脏，这可能是由于化合物的亲脂性。在富含胃泌素释放肽受体的胰腺中也观察到特异性的高摄取。因此，通过引入间隔基团，可在不影响亲和力的情况下改善化合物的体内性质。
