Evaluation of binding of cytotoxic analogs of luteinizing hormone-releasing hormone to human breast cancer and mouse MXT mammary tumor.
The binding characteristics of several cytotoxic analogs of luteinizing hormone-releasing hormone (LH-RH) developed in our laboratory were examined in membranes from human breast cancer and estrogen independent MXT mammary cancer. Specific binding of [125I]D-Trp6-LH-RH and the cytotoxic LH-RH analog [125I]T-98 ([D-Lys6]LH-RH coupled to glutaryl-2-(hydroxymethyl)anthraquinone) (HMAQG) was demonstrated in membrane preparations from human breast and MXT mammary tumor cells. Ligand binding of T-98 was specific, saturable, and dependent on temperature, time, and plasma membrane concentration. Analysis of the binding data showed that in human breast cancer, interaction of [125I]T-98 was consistent with the presence of two classes of LH-RH receptors, one class showing high affinity and low capacity, and the other class showing low affinity and high capacity binding. In membranes from MXT mammary cancer, T-98 bound to one class of saturable, specific, noncooperative binding sites with high affinity and low capacity. The rates of association and dissociation for [125I]T-98 were calculated to be 4.757 x 10(8) M-1 min-1 and 0.016 min-1 (t1/2 = 38.7) in membranes from MXT mammary cancer. In human breast cancer, association rate constants (K1a and K1b) were 2.3 x 10(6) M-1 min-1 for binding to high affinity and 1.8 x 10(4) M-1 min-1 for binding to low affinity binding sites. Dissociation rate constants were K-1a = 0.0801 min-1 (t1/2a = 63.4 min) and K-1b = 0.0467 min-1 (t1/2b = 23.5 min), respectively. [125I]T-98 was not displaced by either unlabeled somatostatin or epidermal growth factor, but was displaced completely by unlabeled T-98 or [D-Trp6]LH-RH. The analysis of displacement curves of [D-Trp6]LH-RH by cytotoxic agonists and antagonists of LH-RH synthesized in our laboratory showed that T-121, AJ-11, T-120, T-133, and T-98 were the most potent in displacing [125I]D-Trp6-LH-RH from breast and MXT cancer membranes. Binding kinetics and analyses of displacement curves of [125I]D-Trp6-LH-RH and [125I]T-98 in membranes of human breast cancer and estrogen independent MXT mouse mammary cancer suggest that binding of the cytotoxic analog T-98 to the LH-RH receptor proceeds reversibly like that of its congeners without cytotoxic radicals. Our findings may provide a stimulus for further studies with LH-RH analogs carrying cytotoxic radicals.(ABSTRACT TRUNCATED AT 400 WORDS)
Milovanovic
S R
SR
Endocrine, Polypeptide and Cancer Institute, Veterans Affairs Medical Center, New Orleans, LA 70146.
Radulovic
S
S
Schally
A V
AV
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D001943:Breast Neoplasms / Q000378:metabolism*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000031:analogs & derivatives / Q000378:metabolism*; D006801:Humans; D015674:Mammary Neoplasms, Animal / Q000378:metabolism*; D051379:Mice; D008875:Middle Aged; D009376:Neoplasms, Hormone-Dependent / Q000378:metabolism; D013696:Temperature; D013997:Time Factors; D017329:Triptorelin Pamoate / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D017329:Triptorelin Pamoate; D007987:Gonadotropin-Releasing Hormone; C035029:LHRH, Lys(6)-