Quadri S M, Vriesendorp H M
Center for Scientific Review, National Institutes of Health, Bethesda, Maryland 20892-7804, USA.
Q J Nucl Med. 1998 Dec;42(4):250-61.
Radiolabeled monoclonal antibodies reactive with tumor-associated antigens can selectively deliver cytotoxic or diagnostic isotopes to malignant cells in vivo. To achieve maximum retention of radiolabel in tumor and a more rapid clearance of radioisotope from normal tissues, six linker immunoconjugates were evaluated in studies using nude mice and beagle dogs. All radioimmunoconjugates contained a mouse monoclonal IgG (QCI) reactive with human ferritin. Different chemical linkages were inserted between immunoglobulins and the radiolabeled chelate (DTPA). Three linkers (ITCB, DSS and BSOCOES) were stable in in vitro and in vivo studies. Three linkers (EGS, DST and DSP) were labile in in vitro and in vivo studies. Indium-111 labeled antiferritin-containing ITCB or DSS linker showed high uptake in human hepatoma xenografts in nude mice. In addition, long blood half-lives and higher normal liver uptakes were noted. Studies of whole body retention of radioimmunoconjugates showed approximately three-fold faster elimination of radioimmunoconjugates containing a labile linker (EGS). EGS linker is the labile linker with the highest therapeutic ratio: higher tumor uptake, but low normal liver uptake and a shortened blood half-life of the radioimmunoconjugate. The differences in normal tissue uptake (liver) between EGS and ITCB were confirmed in beagle dogs. Urine elimination studies and incubation or radioimmunoconjugates in serum or tissue homogenates of tumor, liver or muscle, showed that enzymes in serum and liver homogenates were able to cleave the labile linkers, which led to a more rapid elimination of low molecular weight radioactive metabolites in urine. The metabolism of linker radioimmunoconjugates in tumor was less effective. The labile linker DSP appears less useful because sulphydryl groups that are generated by cleavage of cause higher uptake radioactivity in normal kidney. Biodistribution studies in nude mice were confirmed by serial immunoscintigraphy studies on individual mice. The immunoscintigraphy studies are semi-quantitative only, but enable the use of lower numbers of experimental animals. This is of particular significance in large experimental animals such as beagle dogs. The labile linker approach can reduce normal tissue radiation exposure. The study also provides an example of preclinical optimization of radioimmunoconjugates. Continued use of the appropriate preclinical animal models will accelerate more successful applications of radioimmunoconjugates in cancer patients.
与肿瘤相关抗原反应的放射性标记单克隆抗体能够在体内将细胞毒性或诊断性同位素选择性地递送至恶性细胞。为了实现放射性标记在肿瘤中的最大保留以及放射性同位素从正常组织中更快清除，在使用裸鼠和比格犬的研究中对六种连接体免疫缀合物进行了评估。所有放射免疫缀合物均包含与人铁蛋白反应的小鼠单克隆IgG（QCI）。在免疫球蛋白和放射性标记螯合物（二乙三胺五乙酸，DTPA）之间插入了不同的化学连接体。三种连接体（异硫氰酸苄酯基-N-琥珀酰亚胺碳酸酯，ITCB；二硫代双琥珀酰亚胺基辛二酸酯，DSS；N-琥珀酰亚胺基-3-(2-吡啶二硫代)丙酸酯，BSOCOES）在体外和体内研究中均稳定。三种连接体（乙二醇双琥珀酰亚胺琥珀酸酯，EGS；二硫代琥珀酰亚胺基丙酸酯，DST；3,3'-二硫代二丙酸酯，DSP）在体外和体内研究中不稳定。含铟-111标记的、带有ITCB或DSS连接体的抗铁蛋白在裸鼠的人肝癌异种移植瘤中摄取率高。此外，还观察到血液半衰期长和正常肝脏摄取率高。放射免疫缀合物全身保留研究表明，含有不稳定连接体（EGS）的放射免疫缀合物的清除速度快约三倍。EGS连接体是治疗指数最高的不稳定连接体：肿瘤摄取率较高，但正常肝脏摄取率低，且放射免疫缀合物的血液半衰期缩短。在比格犬中证实了EGS和ITCB在正常组织（肝脏）摄取方面的差异。尿液清除研究以及放射免疫缀合物在肿瘤、肝脏或肌肉的血清或组织匀浆中的孵育表明，血清和肝脏匀浆中的酶能够裂解不稳定连接体，这导致尿液中低分子量放射性代谢产物的清除更快。连接体放射免疫缀合物在肿瘤中的代谢效果较差。不稳定连接体DSP似乎用处较小，因为其裂解产生的巯基导致正常肾脏中放射性摄取较高。对裸鼠的生物分布研究通过对个体小鼠进行的系列免疫闪烁扫描研究得到证实。免疫闪烁扫描研究仅为半定量，但可减少实验动物的使用数量。这在比格犬等大型实验动物中尤为重要。不稳定连接体方法可减少正常组织的辐射暴露。该研究还提供了放射免疫缀合物临床前优化的一个实例。持续使用合适的临床前动物模型将加速放射免疫缀合物在癌症患者中更成功的应用。
