Hormone-drug conjugates

Evaluation of antitumor and toxic side effects of mitomycin C–estradiol conjugates

The antitumor and toxic side effects of mitomycin C-estradiol conjugates (EB-glu-MMC and E-glu-MMC) were evaluated in detail for solutions in propylene glycol and suspensions in 10% (v/v) propylene glycol. Tumor growth, body weight and number of leukocytes were examined after i.p. administration to sarcoma 180 solid tumor-bearing mice. Body weight and number of leukocytes were also examined in normal mice after i.p. administration of the solution. In solution dosage forms, the two conjugates had almost the same suppressive effect on tumor growth at 30 mg MMC eq./kg as MMC at 5 mg/kg, did not lower body weight significantly, but reduced the number of leukocytes at 30 mg MMC eq./kg. MMC, lethally toxic at 10 mg, significantly lowered the body weight and leukocyte number. In the suspension dosage forms, these conjugates had a greater suppressive effect on tumor growth at 50 mg MMC eq./kg than MMC at 5 mg/kg, and reduced the body weight and leukocyte number, with E-glu-MMC more toxic than EB-glu-MMC. The presence of the tumor itself influenced the body weight and leukocyte number. However, toxic side effects could be evaluated from the body weight and leukocyte number to almost the same extent between tumor-bearing and normal mice.

Effective treatment of advanced estrogen-independent MXT mouse mammary cancers with targeted cytotoxic LH-RH analogs

Cytotoxic agents linked to hormonal carriers provide new approaches to tumor therapy, and LH-RH receptors expressed by breast cancers can be used for targeting chemotherapeutic compounds. In the present study, large, advanced estrogen-independent MXT mouse mammary cancers were treated with cytotoxic LH-RH analog AN-152 containing doxorubicin (DOX) or AN-207 incorporating superactive derivative 2-pyrrolino-DOX (AN-201). These cytotoxic hybrid molecules were administered once i.v., close to their maximum tolerated doses, at various time intervals after transplantation of tumors. The cytotoxic LH-RH analogs and the radicals alone, given at earlier stages of tumor development, inhibited growth of MXT cancers. Cytotoxic LH-RH conjugate AN-207 had significantly stronger effect than its respective cytotoxic radical, particularly when larger tumors were treated, causing 95%, 89%, 100% and 96% tumor growth reduction when administered on days 1, 7, 10 or 14, respectively. AN-152, AN-201, and DOX, given on day 14, were virtually ineffective. Histological characteristics of tumor cell proliferation and cell death were analyzed in large MXT cancers 1-4 days after treatment with AN-207 and AN-201. AgNOR scores were decreased and apoptotic indices increased after treatment of tumors with AN-207 or AN-201, but enhanced apoptosis and decreased AgNOR numbers persisted longer in the case of AN-207. In contrast to AN-201, AN-207 also increased the extent of necrosis in tumors. In conclusion, on the basis of its powerful inhibitory effect on the aggressive MXT mouse mammary tumor, the cytotoxic LH-RH analog AN-207 could be considered for treatment of advanced human mammary carcinomas that express LH-RH receptors.

Targeted delivery of chemotherapeutics: tumor-activated prodrug therapy

The potential of targeted delivery of chemotherapeutic drugs for the treatment of cancer has not yet been realized owing to the difficulty of delivering therapeutic concentrations to the target site. While in vivo studies in animal tumor models have produced very encouraging results, clinical studies with antibody-drug conjugates have been less successful. This paper will review the current status of the targeted delivery approach and analyze some of the reasons for the lack of success so far. Starting with a historical perspective, this review will end with a description of newer, more potent and specific antibody-drug conjugates, which behave like tumor-activated prodrugs that may yet fulfil the promise of the targeted delivery approach for the treatment of cancer.

LH-RH analogue carrying a cytotoxic radical is internalized by rat pituitary cells in vitro

The binding and internalization of a cytotoxic analogue of luteinizing hormone-releasing hormone (LH-RH), T-98 (agonist [D-Lys6]LH-RH linked to glutaryl-2-(hydroxymethyl)anthraquinone), by rat anterior pituitary cells was investigated. Analogue T-98 was bound to pituitary membrane binding sites for LH-RH with a high affinity (Kd = 1.2 nM) and was 17 times more potent in releasing luteinizing hormone (LH) from superfused rat pituitary cells than LH-RH. The labeling of this cytotoxic LH-RH analogue was carried out both with radioactive (125I) and nonradioactive iodine. Monoiodination of the Tyr5 residue of T-98 did not significantly affect its binding affinity but greatly decreased its LH-releasing activity to about 3% of the original value. Di-iodination in the same position lowered binding affinity twenty-threefold and further diminished LH-releasing potency. [125I]T-98 was found to bind very strongly to polystyrene, which precluded the use of regular tissue culture plasticware in our experiments. In pituitary cells cultured in glass vials, binding and internalization of [125I]T-98 were observed, which were time and temperature dependent, and which could be inhibited by excess unlabeled analogue. No enzymatic degradation of labeled T-98 was detected in the culture medium during the incubation. Our results indicate that T-98 is internalized by pituitary gonadotropes through receptor-mediated endocytosis. Because this new class of compounds was designed as anticancer drugs, our findings also suggest that this cytotoxic LH-RH agonist may also be internalized by LH-RH receptors present in breast, prostate, ovarian, and other tumors.

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

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.

Polymeric drug-carriers containing doxorubicin and melanocyte-stimulating hormone: in vitro and in vivo evaluation against murine melanoma

N-(2-Hydroxypropyl)methacrylamide (HPMA) copolymers containing doxorubicin (DOX, approximately 8% by weight) bound via the lysosomally degradable spacer Gly-Phe-Leu-Gly and, in certain cases, also melanocyte-stimulating hormone (MSH, 5-10% by weight) were synthesized with the aim of developing a drug conjugate for site-specific delivery to malignant melanoma. Polymer-bound MSH, like free MSH, was able to stimulate tyrosinase activity in B16F10 cells in vitro, confirming the ability of conjugated hormone to interact with the MSH receptor. Similarly, a 125I-labelled conjugate containing MSH was captured by B16F10 cells in vitro more rapidly than a similar polymer without the targeting moiety. HPMA copolymers containing DOX bound via the lysosomally degradable Gly-Phe-Leu-Gly linkage were cytotoxic to a mouse melanoma cell line (M3 S91) in vitro, the MSH-containing conjugate being more active than that without (although the difference in the ID50 was not significant). When administered intraperitoneally or intravenously to C57BL/6J mice bearing intraperitoneal B16F10 tumours, HPMA copolymers containing DOX linked via this biodegradable spacer (with or without MSH) significantly increased animal survival, the maximum ratio of the mean survival of the test group (T) to that of the untreated control (C) T/C observed (approximately 200) over the dose range 5-20 mg DOX/kg being similar to that seen for free DOX. In contrast, neither polymer conjugates containing DOX bound via a non-degradable linkage (Gly-Gly) nor free MSH showed antitumour activity. In mice bearing established subcutaneous B16F10 tumours, biodegradable polymer-bound DOX conjugates given intraperitoneally were more effective than free DOX (which was virtually inactive in this system); conjugates containing MSH were significantly more effective than those without, the maximum T/C being approximately 148 and 324 respectively. Preliminary pharmacokinetic experiments showed evidence of selective MSH targeting of polymer conjugates to subcutaneous B16F10.

Inhibition of growth of experimental prostate cancer in rats by LH-RH analogs linked to cytotoxic radicals

The effects of hybrid cytotoxic LH-RH analogs, produced by linking anthraquinone or methotrexate to carrier LH-RH agonist [D-Lys6]LH-RH, were evaluated in Copenhagen-Fisher F1 rats bearing Dunning R-3327H prostate adenocarcinoma. The two cytotoxic LH-RH analogs T-98 [(D-Lys6)LH-RH coupled to glutaryl-2-(hydroxymethyl)anthraquinone (G-HMAQ)], and AJ-04 [(D-Lys6)LH-RH linked to methotrexate (MTX)], carrier [D-Lys6]LH-RH, or the free cytotoxic compounds MTX and G-HMAQ were administered from Alzet Osmotic minipumps for 7-8 weeks. The cytotoxic LH-RH analogs caused somewhat greater tumor growth inhibition than the carrier peptide, while anthraquinone or methotrexate alone, administered in equimolar doses, were ineffective. The inhibition of androgen sensitive organs (testes, ventral prostates, and seminal vesicles) was pronounced with both carrier and cytotoxic analogs, showing the latter to be fully hormonally active in suppressing the pituitary-gonadal axis. Histological changes were also evaluated. The inhibition of mitosis and the frequency of apoptosis were higher in tumors treated with AJ-04, T-98, [D-Lys6]LH-RH, or by castration than in those of controls. Serum hormone levels were lowered by both carrier peptide and cytotoxic analogs, LH being substantially depressed, and testosterone not detectable. These results and other findings indicate that LH-RH analogs containing cytotoxic radicals anthraquinone or methotrexate retain their hormonal activity after administration in vivo, and can effectively inhibit tumor growth. Extensive further studies are required on this new class of compounds, but apparent binding of cytotoxic LH-RH analogs to tumors such as prostate cancer, which have receptors for LH-RH, could greatly reduce peripheral toxicity of chemotherapeutic agents. This approach, based on targeted chemotherapy, might be of practical therapeutic importance for the management of advanced prostate cancers, which eventually relapse after palliative hormonal therapy.

Cytotoxic analog of somatostatin containing methotrexate inhibits growth of MIA PaCa-2 human pancreatic cancer xenografts in nude mice

Nude mice bearing xenografts of MIA PaCa-2 human pancreatic cancer cell line were treated for 4 weeks with AN-51, a somatostatin octapeptide analog D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Thr-NH2 (RC-121) containing methotrexate attached to the alpha-amino group of D-Phe in position 1. Control groups of mice received saline, RC-121 or methotrexate. Drugs were given in equimolar doses by daily s.c. injections. After 7 days of treatment with 25 micrograms/day of AN-51, tumor growth was completely inhibited although the treatment had to be suspended because of toxic side effects, especially on the gastrointestinal tract, accompanied by major weight loss of the animals. Mice were allowed to recover for 1 week and treatment was continued with 12.5 micrograms/day AN-51. After 2 weeks of additional therapy, tumor volume, percentage change in tumor volume, and tumor weights were significantly decreased, compared with controls, only in the group treated with AN-51. Methotrexate and RC-121 also inhibited tumor growth, but their effects were not statistically significant. AN-51 retained its hormonal activity and decreased serum growth hormone levels in mice. Binding affinity of AN-51 for somatostatin receptors on MIA PaCa-2 cells was found to be 2.5-times lower than that of parent compound RC-121. This is the first report on inhibition of human pancreatic cancer growth in vivo by somatostatin analogs carrying cytotoxic radicals.

Analogues of luteinizing hormone-releasing hormone containing cytotoxic groups

In an attempt to produce better cytotoxic analogues, chemotherapeutic antineoplastic radicals including an alkylating nitrogen mustard derivative of D-phenylalanine (D-melphalan), reactive cyclopropane, anthraquinone derivatives [2-(hydroxymethyl)anthraquinone and the anticancer antibiotic doxorubicin], and an antimetabolite (methotrexate) were coupled to suitably modified agonists and antagonists of luteinizing hormone-releasing hormone (LH-RH). Analogues with D-lysine6 and D-ornithine6 or N epsilon-(2,3-diaminopropionyl)-D-lysine and N delta-(2,3-diaminopropionyl)-D-ornithine were used as carriers for one or two cytotoxic moieties. The enhanced biological activities produced by the incorporation of D amino acids into position 6 of the agonistic analogues were further increased by the attachment of hydrophobic cytotoxic groups, resulting in compounds with 10-50 times higher activity than LH-RH. Most of the monosubstituted agonistic analogues showed high affinities for the membrane receptors of human breast cancer cells, while the receptor binding affinities of peptides containing two cytotoxic side chains were lower. Antagonistic carriers [Ac-D-Nal(2)1,D-Phe(4Cl)2,D-Trp3,Arg5,D-Lys6,D-Ala10] LH-RH [where Nal(2) is 3-(2-naphthyl)alanine], [Ac-D-Nal(2)1,D-Phe(4Cl)2,D-Trp3,Arg5,N epsilon-(2,3-diaminopropionyl)-D-Lys6,D-Ala10]LH-RH, and their D-Pal(3)3 homologs [Pal(3) is 3-(3-pyridyl)alanine] as well as [Ac-D-Nal(2)1,D-Phe(4Cl)2,D-Pal(3)3,Tyr5,N epsilon-(2,3-diamino-propionyl)-D-Lys6,D-Ala10]LH-RH were linked to cytotoxic compounds. The hybrid molecules inhibited ovulation in rats at doses of 10 micrograms and suppressed LH release in vitro. The receptor binding of cytotoxic analogues was decreased compared to the precursor peptides, although analogues with 2-(hydroxymethyl)anthraquinone hemiglutarate had high affinities. All of the cytotoxic analogues tested inhibited [3H]thymidine incorporation into DNA in cultures of human breast and prostate cancer cell lines. Some cytotoxic analogues also significantly suppressed the growth of mammary and prostate cancers in vivo in animal models.

Increased affinity of insulin for its receptor following conjugation to a second protein

It has been observed that various glutaraldehyde linked conjugates of insulin and bovine serum albumin display enhanced binding to the insulin receptor. High molecular weight covalent complexes resulting from the conjugation of insulin to both N-acylurea albumin and unmodified albumin have been shown to displace (125I-TyrA14) human insulin from HepG2 cell receptors successfully. Results indicate that their affinity for the insulin receptor is greater than that of insulin itself by an order of magnitude. The relationship between these results and similar insulin binding phenomena reported by other workers ('super-activity') is examined, particularly with regard to possible alterations to the insulin binding site brought about by the respective conjugation procedures. It is suggested that modifications to the B29 lysine residue might play a crucial role in stabilising the interaction of conjugated insulin with its cognate receptor. The powerful potential of insulin to act as carrier in the intracellular delivery of drugs and other molecules is discussed.