Effect of treatment with LHRH analogs containing cytotoxic radicals on the binding characteristics of receptors for luteinizing-hormone-releasing hormone in MXT mouse mammary carcinoma

Abstract

Binding capacities and apparent dissociation constants of receptors for luteinizing-hormone-releasing hormone (LHRH) were investigated in estrogen-independent MXT mammary cancers of untreated mice and after in vivo treatment with agonistic or antagonistic analogs of LHRH containing cytotoxic radicals: AJ-04 (agonist [D-Lys6]LHRH linked to methotrexate), T-98-([D-Lys6]LHRH coupled to glutaryl-2-(hydroxmethyl)anthraquinone (G-HMAQ)) and T-121/B (LHRH antagonist T-147 containing two residues of G-HMAQ), which induced tumor growth inhibition. The effects were compared to LHRH agonist [D-Trp6]LHRH and carriers [D-Lys6]LHRH, LHRH antagonist T-147, as well as to methotrexate, G-HMAQ and surgical bilateral overiectomy. Analysis of the binding data revealed that in control tumors the interaction of 125I-[D-TRP6]LHRH was consistent with the presence of one class of saturable, specific, noncooperative, high-affinity and low-capacity binding sites. Chronic treatment of mice bearing MXT tumors with LHRH analogs AJ-04 and T-121/B carrying cytotoxic radicals, but not with T-98 produced significant down-regulation of membrane receptors for LHRH. The largest decrease in dissociation binding constant and Bmax of receptors for LHRH was also found in animals treated with T-121/B. Specific, high affinity binding of 125I-labelled epidermal growth factor (EGF) was detected in the membranes from control and treated MXT tumors. Treatment with cytotoxic LHRH analogs, AJ-04, T-98 and especially with T-121/B, reduced maximal binding capacity of EGF receptors. Our results indicate that LHRH analogs carrying cytotoxic radicals retain their hormonal activity and inhibit tumor growth while inducing down-regulation of LHRH receptors. In addition, probably both components of the cytotoxic LHRH analog, peptide carriers and cytotoxic radicals, reduce the binding capacity of EGF receptors, which might be useful in the treatment of breast cancer.

Effect of a cytotoxic analog of LH-RH (T-98) on the growth of estrogendependent MXT mouse mammary cancers: Correlations between growth characteristics and EGF receptor content of tumors

“…In this study we also showed that LH-RH analogs decreased binding capacity of EGF receptors on tumor cells. This finding is in agreement with previous studies [30]. The cytotoxic analog was more effective than [D-Lys6]LH -RH in producing a down-regulation of EGF receptors.…”

“…These receptors can mediate antitumor effects of LH-RH analogs. In several earlier studies, we have shown the presence of receptors for LH-RH in MXT breast cancers [12,13,30]. In this study, the finding that labeled T-98 accumulated in MXT tumors 3 hours after s.c. injection shows that specific targeting might play a role in the enhanced effect of cytotoxic analog T-98.…”

Evaluation of binding of cytotoxic analogs of luteinizing hormone-releasing hormone to human breast cancer and mouse MXT mammary tumor

“…Binding affinity of cytotoxic LH-RH analog T-98 was essentially identical in both equilibrium and kinetic experiments. Equilibrium homologous displacement of [D-Trp6]LH-RH and T-98 using human breast tumor membranes confirmed present kinetic data as well as our previous results [23,24,27] that [DTrp~]LH-RH in human breast cancer membranes binds to two classes of receptors. In rat pituitary superfusion systems T-98 was shown to be a powerful analog in a wide range of doses [19].…”

“…Receptors for LH-RH were previously found in human breast cancer as well as in estrogen dependent and estrogen independent MXT mammary cancers [12,13,15,[23][24][25][26][27][28]. Our kinetics experiments presented herein, suggest that cytotoxic LH-RH analogs bind to two classes of receptors in membranes of human breast cancer.…”

“…Antagonistic carriers [Ac-DNal (2) [19]. In our previous studies, we showed that cytotoxic LH-RH analogs AJ-004, T-98 and T-121/B caused a significant inhibition of tumor growth and, except for T-98, down-regulation of membrane receptors for [D-Trp6]LH-RH in estrogen independent MXT mammary tumor membranes [23,24]. LH-RH analogs carrying cytotoxic radicals retain their endocrine activity after administration in vivo and can apparently be bound to tumors which have receptors for LH-RH [23].…”