Groothuis D R, Lippitz B E, Fekete I, Schlageter K E, Molnar P, Colvin O M, Roe C R, Bigner D D, Friedman H S
Department of Neurology, Northwestern University Medical School, Evanston Hospital, Illinois 60201.
Cancer Res. 1992 Oct 15;52(20):5590-6.
Melphalan (L-phenylalanine mustard, L-PAM, alkeran; molecular weight, 305,000) is transported across tumor cell membranes and the blood-brain barrier by the large neutral amino acid (LNAA) transport system. Normally, plasma LNAA levels are high enough and the affinity low enough that this system does not transport much melphalan into the brain. However, plasma amino acids can be reduced by fasting and protein-free diet. We used this method to reduce competition and to increase melphalan transport into brain tumors. In nude mice fasted for 12 h and then fed a protein-free diet for 2 and 6 h, mean plasma LNAA levels were 46% and 42% of control values. Nude mice with xenotransplanted D-54MG human gliomas were used to study tissue distribution and uptake kinetics of [3H]melphalan in a control group and a diet group (after a 12-h fast and 2 h of a 0% protein diet). The K1 (blood-to-tissue transfer constant) of melphalan, determined by graphical analysis and by nonlinear fitting to a 2-compartment model, was higher in the diet group in all tumor regions except the necrotic center of subcutaneous tumors; the increase was significant in the tumor periphery of brain and s.c. tumors. The ratio of K1s (diet to control) varied from 1.2 to 1.3 in brain tumors, 1.9 to 2.1 in subcutaneous tumors, and 1.8 to 3.1 in tumor-free brain. The apparent [3H]melphalan distribution space was significantly higher in the tumor periphery of both brain and subcutaneous tumors of the 15- and 30-min diet group. We also measured blood-brain barrier transport of [alpha-14C]aminoisobutyric acid and blood flow (with [131I]iodoantipyrine): the K1 of [alpha-14C]aminoisobutyric acid was 28.1 +/- 6.6 (SE) in brain tumors and 24.3 +/- 8.9 microliters/g/min in subcutaneous tumors. Blood flow was 58.2 --> 3.9 in brain tumors and 5.2 +/- 0.4 ml/100 g/min in subcutaneous tumors. Fasting, when combined with a protein-free diet, reduces plasma amino acid levels and thereby reduces competition between melphalan and LNAAs. This may increase the amount of melphalan that can enter a brain tumor without increasing the administered drug dose and suggests a therapeutic manipulation that can be used to increase the delivery of melphalan.
美法仑（L-苯丙氨酸氮芥、L-PAM、马法兰；分子量305,000）通过大中性氨基酸（LNAA）转运系统穿过肿瘤细胞膜和血脑屏障。正常情况下，血浆中LNAA水平足够高而亲和力足够低，以至于该系统不会将大量美法仑转运至脑内。然而，禁食和无蛋白饮食可降低血浆氨基酸水平。我们采用这种方法来减少竞争并增加美法仑向脑肿瘤的转运。在禁食12小时然后给予无蛋白饮食2小时和6小时的裸鼠中，平均血浆LNAA水平分别为对照值的46%和42%。将移植了D-54MG人胶质瘤的裸鼠用于研究[³H]美法仑在对照组和饮食组（禁食12小时并给予0%蛋白饮食2小时后）中的组织分布和摄取动力学。通过图形分析和对二室模型进行非线性拟合确定的美法仑的K1（血-组织转移常数），在饮食组中，除皮下肿瘤坏死中心外的所有肿瘤区域均较高；在脑肿瘤和皮下肿瘤的肿瘤周边，升高具有显著性。脑肿瘤中K1s（饮食组与对照组）的比值在1.2至1.3之间，皮下肿瘤中为1.9至2.1，无肿瘤脑内为1.8至3.1。在15分钟和30分钟饮食组的脑肿瘤和皮下肿瘤的肿瘤周边，[³H]美法仑的表观分布容积均显著更高升高。我们还测量了[α-¹⁴C]氨基异丁酸的血脑屏障转运和血流量（用[¹³¹I]碘安替比林）：脑肿瘤中[α-¹⁴C]氨基异丁酸的K1为28.1±6.6（SE），皮下肿瘤中为24.3±8.9微升/克/分钟。脑肿瘤中的血流量为58.2→3.9，皮下肿瘤中为5.2±0.4毫升/100克/分钟。禁食与无蛋白饮食联合使用时，可降低血浆氨基酸水平，从而减少美法仑与LNAA之间的竞争。这可能会增加在不增加给药剂量的情况下能够进入脑肿瘤的美法仑量，并提示一种可用于增加美法仑递送的治疗操作方法。
