Blakey D C, Skilleter D N, Price R J, Watson G J, Hart L I, Newell D R, Thorpe P E
Drug Targeting Laboratory, Imperial Cancer Research Fund, Lincoln's Inn Fields, London, England.
Cancer Res. 1988 Dec 15;48(24 Pt 1):7072-8.
Immunotoxins containing the ribosome-inactivating protein, saporin, are very effective antitumor agents but are highly toxic to mice. They induce severe necrotic lesions in the liver parenchyma of the recipients. Such extensive damage to the liver parenchyma is not observed with ricin A-chain immunotoxins even at 5-fold higher dosage. The hepatotoxicity of the saporin immunotoxins was found in the present study to arise from a combination of two effects. First, saporin and saporin immunotoxins were 30- and 6-fold more toxic to primary cultures of mouse liver parenchymal cells than were ricin A-chain and ricin A-chain immunotoxins, respectively. This was despite the fact that the cells bound 4- to 5-fold less saporin or saporin immunotoxins than ricin A-chain or ricin A-chain immunotoxins. The binding of ricin A-chain and its immunotoxin to the cells was mediated through the carbohydrate residues present on the A-chain whereas saporin is not glycosylated and thus must bind to other sites on the cell surface which result in transport of saporin relatively efficiently to the cytosol. The second reason for the hepatotoxic action of the saporin immunotoxin was that it had a longer blood half-life (t 1/2 alpha = 1.1 h; t 1/2 beta = 17.1 h) than the ricin A-chain immunotoxin (t 1/2 = 0.52 h; t 1/2 beta = 9.7 h). Analyses using a two-compartment pharmacokinetic model showed that the two immunotoxins broke down in vivo to give free antibody at a similar rate (t 1/2 = 10-12 h) but that the ricin A-chain immunotoxin was eliminated 11 times more rapidly than the saporin immunotoxin by routes other than breakdown. It was calculated that, in mice given a median lethal dose of saporin immunotoxin, the blood levels of immunotoxin remained above the concentration that killed 50% of parenchymal cells in vitro for more than 48 h. In mice given a median lethal dose of ricin A-chain immunotoxin, the blood levels fell below the concentration that was toxic to parenchymal cells in vitro within 4 h. The longer blood half-life of the saporin immunotoxin may also explain our previous finding that it had antitumor activity superior to that of a ricin A-chain immunotoxin in mice.
含有核糖体失活蛋白皂草素的免疫毒素是非常有效的抗肿瘤药物，但对小鼠具有高毒性。它们会在受体的肝实质中诱导严重的坏死性病变。即使剂量高出5倍，蓖麻毒素A链免疫毒素也不会导致如此广泛的肝实质损伤。本研究发现，皂草素免疫毒素的肝毒性源于两种效应的结合。首先，皂草素和皂草素免疫毒素对小鼠肝实质细胞原代培养物的毒性分别比蓖麻毒素A链和蓖麻毒素A链免疫毒素高30倍和6倍。尽管细胞结合的皂草素或皂草素免疫毒素比蓖麻毒素A链或蓖麻毒素A链免疫毒素少4至5倍。蓖麻毒素A链及其免疫毒素与细胞的结合是通过A链上存在的碳水化合物残基介导的，而皂草素没有糖基化，因此必须与细胞表面的其他位点结合，这导致皂草素相对有效地转运到细胞质中。皂草素免疫毒素肝毒性作用的第二个原因是它的血液半衰期更长（t 1/2α = 1.1小时；t 1/2β = 17.1小时）比蓖麻毒素A链免疫毒素（t 1/2 = 0.52小时；t 1/2β = 9.7小时）。使用二室药代动力学模型的分析表明，两种免疫毒素在体内分解产生游离抗体的速率相似（t 1/2 = 10 - 12小时），但蓖麻毒素A链免疫毒素通过分解以外的途径消除速度比皂草素免疫毒素快11倍。据计算，给予皂草素免疫毒素半数致死剂量的小鼠，免疫毒素的血液水平在体外杀死50%实质细胞的浓度以上保持超过48小时。给予蓖麻毒素A链免疫毒素半数致死剂量的小鼠，血液水平在4小时内降至对体外实质细胞有毒的浓度以下。皂草素免疫毒素更长的血液半衰期也可能解释我们之前发现的它在小鼠中具有优于蓖麻毒素A链免疫毒素的抗肿瘤活性。
