Hormonotoxins

Folate receptor mediated targeted delivery of ricin entrapped into sterically stabilized liposomes to human epidermoid carcinoma (KB) cells: effect of monensin intercalated into folate-tagged liposomes

Ricin was encapsulated into various sterically stabilized liposomes having different density of folate on the surface and the cytotoxicity of ricin in these liposomes was examined in KB cells. The effect of monensin in free and various sterically stabilized liposomal forms having different density of folate on the surface on the enhancement of cytotoxicity of ricin entrapped in these liposomes was also examined. It was observed that liposomal ricin having 0.5 mol% folate-PEG on the surface exhibits maximum cytotoxicity (IC(50)=1274 ng/ml) in KB cells as compared to non-targeted liposomes (IC(50)=3274 ng/ml). Monensin either in free form (266.2-fold) or liposomal form (291.5-fold) enhances the cytotoxicity of this targeted liposomal ricin significantly. This enhancement of the cytotoxicity of ricin entrapped in folate-targeted liposomes is further enhanced to 557.7-fold by monensin when it was delivered through folate-targeted (0.5 mol% folate-PEG) liposomes. The present study has clearly demonstrated that ricin entrapped in folate-tagged-sterically stabilized liposomes in combination with monensin intercalated in folate-tagged-sterically stabilized liposomes may have potential application for the treatment of cancer cells over-expressing folate receptors on the cell surface.

Enhanced killing of human epidermoid carcinoma (KB) cells by treatment with ricin encapsulated into sterically stabilized liposomes in combination with monensin

Ricin was encapsulated in various charged liposomes having 5 mol% PEG of different chain length on the surface. The cytotoxicity of ricin entrapped in these liposomal formulations was examined in human epidermoid carcinoma (KB) cells with a view to develop an optimum delivery system for ricin in vivo. It was observed that the cytotoxicity of ricin entrapped in various charged liposomes was significantly dependent on the surface charge as well as chain length of PEG. The maximum cytotoxicity of ricin was observed when it was delivered through negatively charged liposomes having 5 mol% PEG-2000 on the surface. Monensin enhances the cytotoxicity of ricin entrapped in various charged liposomes depending on the surface charge. Maximum potentiation of cytotoxicity of ricin was observed when it was delivered through negatively charged liposomes having 5 mol% PEG-2000 on the surface. Studies on the kinetics of inhibition of protein synthesis by ricin revealed that the lag period of inhibition of protein synthesis is significantly lengthened following its delivery through various charged liposomes. Monensin significantly reduced the lag period of action of ricin. It was also observed that the efficacies of monensin on the enhancement of cytotoxicity of ricin entrapped in various charged PEG-liposomes were highly related to their amount of cell association. The current study has demonstrated that by suitable adjustment of charge, density, and chain length of PEG on the surface of liposomes it would be possible to direct liposomal ricin to human tumor cells for their selective elimination in combination with monensin.

Effect of surface charge and density of distearylphosphatidylethanolamine-mPEG-2000 (DSPE-mPEG-2000) on the cytotoxicity of liposome-entrapped ricin: Effect of lysosomotropic agents

Ricin was encapsulated in various liposomes having neutral, negatively and positively charged and different density of DSPE-mPEG-2000 on the surface and cytotoxicity of ricin entrapped in these different charged liposomal formulations was studied in CHO pro(-) cells and compared with free ricin with a view to develop an optimum delivery system for ricin in vivo. It was observed that the cytotoxicity of ricin entrapped in various charged liposomes was significantly dependent on the charge on the surface of liposomes. The maximum cytotoxicity of ricin was observed when it was delivered through negatively charged liposomes. Monensin enhances the cytotoxicity of ricin entrapped in various charged liposomes and the extent of enhancement of the cytotoxicity is significantly dependent on the charge on the surface of liposomes. Maximum potentiation (213.14-fold) of cytotoxicity of ricin was observed when it was delivered through positively charged liposomes followed by negatively charged (83.36-fold) and neutral (71.30-fold) liposomes, respectively. Studies on the kinetics of inhibition of protein synthesis by ricin entrapped in various charged liposomes revealed that lag period of inhibition of protein synthesis is significantly lengthened following delivery through various charged liposomes. However, in the presence of monensin, the lag period was reduced. There is a marginal variation in the cytotoxicity of ricin entrapped in various charged liposomes after incorporation of 5mol% of DSPE-mPEG-2000 on the surface. However, there is a significant variation in the enhancing potency of monensin on the cytotoxicity of ricin entrapped in various charged liposomes in CHO pro(-) cells following incorporation of 5mol% DSPE-mPEG-2000 on the surface. Studies on the effect of variation of density of DSPE-mPEG-2000 on the surface of various charged liposomes on the enhancement of cytotoxicity of entrapped ricin by monensin in CHO pro(-) cells showed that the enhancing potency of monensin on the cytotoxicity of ricin entrapped in various charged liposomes is significantly dependent on the density of DSPE-mPEG-2000 on their surface. It was also observed that the efficacies of monensin on the enhancement of cytotoxicity of ricin entrapped in various charged PEG-liposomes in CHO pro(-) cells was highly related to their amount of cell-association. The present study has clearly shown that by suitable alteration of liposomal lipid composition, charge and density of hydrophilicity it would be possible to direct liposomal ricin to specific cells for their selective elimination in combination with monensin.

Enhancement of the cytotoxicity of liposomal ricin by the carboxylic ionophore monensin and the lysosomotropic amine NH4Cl in Chinese hamster ovary cells

Ricin was encapsulated in negatively charged liposomes and its effect on the cytotoxicity was compared with native ricin in Chinese hamster ovarian (CHO) cells. The cytotoxicity of ricin, as measured by a marker protein synthesis (incorporation of 3H-leucine), was reduced markedly (300-fold) following encapsulation in liposomes. Lactose, a potent inhibitor of ricin cytotoxicity, had no effect on the binding, internalization, and cytotoxicity of liposomal ricin, indicating that liposomal ricin enter into mammalian cells by an alternative route, bypassing galactose-mediated endocytic pathway. Both monensin (a carboxylic ionophore) and NH4Cl (a lysosomotropic amine) markedly enhances the cytotoxicity of liposomal ricin, indicating endocytotic uptake of liposomal ricin. The degree of potentiation of the cytotoxicity of liposomal ricin by both monensin and NH4Cl was significantly higher (441- and 51-fold) as compared to native ricin (62.5- and 12.5-fold). The extent of exocytosis of free ricin was found to be much higher as compared to liposomal ricin; on the other hand, the extent of degradation of free and liposomal ricin was identical. Consequently, the intracellular level of liposomal ricin was increased to 3.5-fold. This higher level of intracellular liposomal ricin may allow more efficient ricin A-chain release into the cytosol under the influence of NH4Cl and monensin. Monensin-induced potentiation of liposomal ricin was prevented by brefeldin A, indicating that in the presence of monensin, the liposomal ricin was efficiently routed through the Golgi apparatus en route to the cytosol. Thus, liposomal ricin in combination with monensin may have potential application for selective elimination of malignant cells.

A Sandwich Enzyme-Linked Immunoabsorbent Assay for Measurement of Gonadotropin-Releasing Hormone-Toxin Conjugates

PROBLEM

Biological effectiveness of targeted cytotoxins is dependent on their stability, circulating half-life, receptor binding ability, and cytotoxicity. The objective of this study was to compare stability of gonadotropin-releasing hormone (GnRH)-toxin conjugates made with disulfide linkers to those using a maleimidodibutyryl (mb) linkage.

METHOD OF STUDY

We developed a sandwich enzyme-linked immunoabsorbent assay recognizing both GnRH analog and cytotoxin to ensure the conjugate measured was intact. Anti-D-Leu(6)-GnRH was used for capture and anti-pokeweed antiviral protein (anti-PAP) or anti-RNase for quantification. Specificity was verified by lack of reactivity with ovine FSH and LH, PAP, RNase, and D-Lys(6)-GnRH.

RESULTS

Conjugates prepared using disulfide linkages were not stable in serum in vitro (half-lives <10 min), whereas mb conjugates had half-lives >2 hr. Clearance of mbGnRH-PAP from the circulation of sheep was rapid (t(1/2) <20 min).

CONCLUSION

The assays were found to be specific, sensitive and accurate for measurement of GnRH-toxin conjugates in vitro and in vivo.

Tumor cell killing enabled by listeriolysin O-liposome-mediated delivery of the protein toxin gelonin

Gelonin is a type I plant toxin that has potential as an effective anti-tumor agent by virtue of its enzymatic capacity to inactivate ribosomes and arrest protein synthesis, thereby effectively limiting the growth of cancer cells. Being a hydrophilic macromolecule, however, gelonin has limited access to its target subcellular compartment, the cytosol; it is effectively plasma membrane-impermeant and subject to rapid degradation within endosomes and lysosomes upon cellular uptake as it lacks the membrane-translocating capability that is typically provided by a disulfide-linked B polypeptide found in the type II toxins (e.g. ricin). These inherent characteristics generate the need for the development of a specialized cytosolic delivery strategy for gelonin as an effective anti-tumor therapeutic agent. Here we describe an efficient means of delivering gelonin to the cytosol of B16 melanoma cells. Gelonin was co-encapsulated inside pH-sensitive liposomes with listeriolysin O, the pore-forming protein that mediates escape of the intracellular pathogen Listeria monocytogenes from the endosome into the cytosol. In in vitro experiments, co-encapsulated listeriolysin O enabled liposomal gelonin-mediated B16 cell killing with a gelonin IC50 of approximately 0.1 nM with an extreme efficiency requiring an incubation time of only 1 h. By contrast, cells treated with equivalent concentrations of unencapsulated gelonin or gelonin encapsulated alone in pH-sensitive liposomes exhibited no detectable cytotoxicity. Moreover, treatment by direct intratumor injection into subcutaneous solid tumors of B16 melanoma in a mouse model showed that pH-sensitive liposomes containing both listeriolysin O and gelonin were more effective than control formulations in curtailing tumor growth rates.

A novel approach of targeted ablation of mammary carcinoma cells through luteinizing hormone receptors using Hecate-CGbeta conjugate

Recent studies have shown that human and animal mammary gland carcinoma cell line express luteinizing hormone receptors (LHRs). We have examined the cytotoxic effect of Hecate-CGbeta conjugate, that is, fusion of a lytic peptide (Hecate) and a 15-amino acid fragment of the CGbeta-chain in vitro. To test the hypothesis that the Hecate-CGbeta conjugate selectively abolishes cells possessing LHR, estrogen dependent and independent human breast cancer cell lines (MCF-7; MDA-MB-231) and a mouse Leydig tumor cell line (BLT-1) were treated in vitro with Hecate-CGbeta conjugate and Hecate alone. Cytotoxic effects of the Hecate-CGbeta conjugate and the Hecate alone was measured by lactate dehydrogenase (LDH) release immediately after treatment. We observed that the Hecate-CGbeta conjugate selectively, in dose-dependent manner destroys cells possessing LHR in lower concentrations of preparate comparing to the Hecate alone and that the cytotoxic effect is strongly correlated with the number of LHR. Using Western blot analysis we characterized the LHR on membranes of MDA-MB-231, MCF-7 and BLT-1 tumor cell lines. In addition, we showed the evaluation of inhibition potential of the Hecate-CGbeta conjugate to LHR. At a concentration of 33 microM the conjugate inhibited (50%; IC50) the binding of CG to LHR. We suggest further development of this novel approach for the treatment of breast cancer by the Hecate-CGbeta for in vivo trials.

Effect of amino group modification of ovine luteinizing hormone (oLH) by N-succinimidyl 6-[3-(2-pyridyldithio)propionate]hexanoate, a long chain N-succinimidyl-3-(2-pyridyldithio) propionate (SPDP) on immunological and biological properties: a comparative study with SPDP modified oLH

The epsilon-NH2 groups of ovine luteinizing hormone has been modified with the long chain N-succinimidyl-3-(2-pyridyl dithiopropionate (LC-SPDP). The LC-SPDP modification primarily occurs in -NH2 groups of the alpha-subunit. Although, the sequential modification of lysine residue in alpha-subunit led to progressive reduction in the receptor binding and immunological properties but the steroidogenic activity was relatively unaffected. The immunoreactivity and receptor binding properties of LC-SPDP modified oLH molecule were more affected comparative to SPDP modified derivatives. This suggested that the increase in hydrophobic carbon chain in LC-SPDP-oLH molecules resulted into the drastic inhibition in the immunological and biological properties. However, the steroidogenic potential of LC-SPDP/or SPDP-oLH derivative was comparable. The present study clearly demonstrate that a single -NH2 group modification with LC-SPDP would generate the site for the conjugation to the toxin/carrier proteins and the resultant oLH-S-S-toxin conjugate would retain significant immunological and biological properties of the hormone molecule.

Hormonotoxins: the role of positive charge of lysine residue on the immunological, biological and cytotoxic properties of ovine lutropin-S-S-gelonin conjugates

Since the positive charge on the lysine residues plays an important role in the receptor recognition ability of oLH, the hormonotoxin has been synthesised with the use of 2-iminothiolane HC1 (2IT) and N-Succinimidyl-3-(2-pyridyldithio)-propionate (SPDP). The oLH activated with 2IT (oLH-10) was then mixed with SPDP activated gelonin (gelonin-30) in order to obtain a oLH-S-S-gelonin hormonotoxin. The conjugation mixture containing hormonotoxin was purified by gel-filtration chromatography according to the molecular weight and a complete physico-chemical, immunochemical and biochemical analysis were performed. The linkage occurred through the epsilon-NH2 groups of alpha-subunit of oLH as judged from RP-HPLC analysis. A1:1 (oLH:gelonin) molar ratio was obtained when determined with the use of several techniques. The hormonotoxins retained substantial receptor binding, steroidogenic activity and immunoreactivity. The competitive displacement analysis indicate that the binding occurs via the hormone part leaving the gelonin free which was probed with the gelonin antibodies. The presently described (C150A-02, C160A-02 and C170A-02) hormonotoxins exhibited higher receptor binding and toxicity to the target cells than the hormonotoxins prepared with the use of SPDP only. Therefore it is concluded that higher receptor binding and cytotoxicity may be due to the retention of positive charge on the lysine residues of oLH which was preserved during the conjugation process.

Disulfide linked alpha oLH-gelonin conjugate failed to recombine with beta oLH subunit to generate bioeffective hormonotoxin

Since, linking of ovine luteinizing hormone (oLH) to ribosome inactivating protein gelonin (in oLH-gelonin conjugate) occur via the alpha-subunit, alpha oLH, an attempt has been made to develop a 'universal' hormonotoxin for selective targeting to specific cells in the gonads. Four different molar ratios of oLH and N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP) were used to activate the epsilon amino (phi-NH2) groups of alpha oLH. The alpha oLH-SPDP derivatives recombine to native beta subunit of oLH (beta oLH) and the purified recombinants retained substantial receptor binding, steroidogenic activity and immunoreactivity to native oLH. The disulfide linked alpha oLH-S-S-gelonin conjugates prepared by SPDP method were purified by gel filtration chromatography and analysed by reverse-phase high performance liquid chromatography (RP-HPLC). In order to obtain specificity and bioeffectivity, the alpha oLH-S-S-gelonin conjugates were allowed to recombine to native beta oLH and the recombination mixture was further purified by gel-filtration chromatography. The RP-HPLC analysis of these recombinants indicated that alpha oLH-S-S-gelonin did not recombine to beta oLH. The failure of recombination may be due to the reasons. (i) The site of epsilon-NH2 activation by SPDP may be different in the alpha oLH than the native oLH. (ii) The activation site may be in close proximity to the annealing site which facilitates the recombination of beta-subunit but failed to reassociate to alpha oLH-S-S-gelonin conjugate.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytotoxic activity of lutropin-gelonin conjugate in mouse Leydig tumor cells: potentiation of the hormonotoxin activity by different drugs

A hormonotoxin preparation composed of gelonin, a basic protein of 30,000 Da isolated from the plant Gelonium multiflorum and the luteinizing hormone (LH, lutropin) isolated from the sheep pituitary has been studied for its cytotoxic action on mouse testicular Leydig tumor cells (MA-10 cells). Gelonin modified with 2-iminothiolane and conjugated with hormone modified by N-succinimidyl-3-2-pyridyl dithiopropionate was able to inhibit protein synthesis in Leydig tumor cells. An enhancement of the cytotoxicity of the hormonotoxin was obtained in the presence of drugs like quinacrine, chloroquine, verapamil and monensin. We report that the cytotoxicity of hormonotoxin was enhanced 10-15 times with quinacrine (7.6 microM), chloroquine (29 microM), verapamil (40 microM) and monensin (0.29 microM). While quinacrine, chloroquine and verapamil were not cytotoxic to MA-10 cells for up to 48 h, monensin alone reduced protein synthesis significantly in 48 h. All the drugs studied here inhibited steroidogenic action of the native hormone even at concentrations which were not detrimental to protein synthesis. On the basis of the above studies, we suggest that it may be feasible to develop combination strategies to destroy gonadal cells bearing gonadotropin (LH) receptors. In cells not bearing LH receptors (COS-7 cell line) there was no cytotoxicity either with hormonotoxin alone or in combination with the drugs, suggesting specificity of action.

A comparison of anti-lymphocyte immunotoxins containing different ribosome-inactivating proteins and antibodies

Immunotoxins were prepared with several single-chain ribosome-inactivating proteins (RIPs type 1) and with the A-chain of ricin linked to the F(ab')2 fragment of sheep anti-mouse IgG. The cytotoxic activity of these conjugates was tested on human lymphocytes pretreated with an anti-CD3 murine MoAb. The immunotoxins inhibited DNA synthesis in phytohaemagglutinin (PHA)-stimulated lymphocytes with IC50S (concentrations causing 50% inhibition) ranging from 8.9 x 10(-13) to 5.7 x 10(-11) M (immunotoxins containing dianthin 32, saporin, pokeweed antiviral protein from seeds (PAP-S), bryodin, momordin, momorcochin, and trichokirin), 1 x 10(-8) M (immunotoxin containing gelonin) and 5 x 10(-9) M (immunotoxin containing ricin A-chain). The immunotoxin containing saporin linked to the anti-mouse IgG F(ab')2 fragment was also highly toxic to human lymphocytes pretreated with anti-CD2, -CD3, -CD5 and -CD45 MoAbs, with IC50S less than or equal to 10(-11) M. Immunotoxins were prepared also with saporin linked to MoAbs against various CD antigens. The immunotoxin prepared with the anti-CD3 antibody had the highest specific cytotoxicity to human lymphocytes.

In vivo potentiation of ricin toxicity by monensin delivered through liposomes

Monensin, a carboxylic ionophore, which is known to raise intravesicular pH, was intercalated in liposomes and its effect on the toxicity of ricin in mice was studied. The toxicity of ricin in vivo was found to be significantly enhanced by the administration of monensin intercalated in liposomes (liposomal monensin). The observed enhancement of the toxicity of ricin by monensin was highly dose-dependent and was maximal when ricin was injected within 60 min of monensin injection. The survival time was found to be reduced in the range of 8-20 h, depending on the dose of ricin used, by liposomal monensin. Stability of liposomes containing monensin as inferred from the release of entrapped calcein or FITC-dextran under both in vivo and in vitro conditions was comparable to that observed for liposomes without monensin. Liposomal monensin remains in circulation for 2 h and was cleared from the blood stream after 4 h. In contrast, 15 min was required for the clearance of monensin when administered in free form. Studies on the distribution of liposomal monensin and 125I-ricin in various tissues have revealed that monensin is mainly localized in the liver and spleen which are also the major sites for ricin accumulation. Our observation on the substantial enhancement of ricin toxicity in vivo by liposomal monensin strongly supports the potential usefulness of the latter as a potentiating agent in the enhancement of the toxicity of immunotoxin or hormonotoxin for selective elimination of cancer cells.

Number and topography of epitopes of human chorionic gonadotropin (hCG) are shared by desialylated and deglycosylated hCG

A previously established map of the surface epitopes of human chorionic gonadotropin (hCG) served as template for the present study in which we investigated the antigenic surfaces of two glycosylation variants of hCG, i.e. desialylated hCG (asialo-hCG) and deglycosylated hCG (degly-hCG). This map allocates five epitopes to the alpha subunit, five to the beta subunit and four alpha beta epitopes to structures formed only by the alpha/beta heterodimer holo-hCG (Schwarz et al. (1986) Endocrinology 118, 189-197; Berger et al. (1990) J. Endocrinol. 125, 301-309). Here it is described that both variants complied with this template: each of the 14 distinct monoclonal antibodies with which the epitopes of hCG were defined reacted with radiolabeled asialo-hCG and degly-hCG as well and generally bound degly-hCG with greater affinity than hCG. Moreover, every combination of capture and radiolabeled detection antibody that was either compatible or incompatible on unlabeled hCG was so also on unlabeled asialo-hCG and degly-hCG. It thus appears that alterations of the carbohydrate structure of hCG can be associated with a change in affinity between some antibodies and their respective epitopes but not with a loss of an epitope or with a change in the topographical relationships of the 14 epitopes.

Blood clearance and organ distribution and tissue concentration of native, homopolymerized and IgG-conjugated ribosome-inactivating proteins

1. The blood clearance, organ distribution and tissue concentration of several native, homopolymerized, and IgG-conjugated 125I-labelled ribosome-inactivating proteins (RIPs) were determined in mice. 2. Native RIPs were cleared rapidly from blood, with half-lives of 4-8 min, and were concentrated mainly in the kidneys. 3. After conjugation to IgG the three RIPs studied showed increased blood half-lives and decreased concentrations in the kidneys. 4. The two homopolymers studied had blood half-lives and kidney concentrations intermediate to those of free and conjugated RIPs. 5. These results indicate that after IgG-conjugation the increased half-lives of the RIPs studied were at least in part due to the larger molecular size of the conjugates and to their lower renal excretion.

Epidermal growth factor labeled beta-amanitin-poly-L-ornithine: preparation and evidence for specific cytotoxicity

Poly-L-ornithine with an average molecular weight of 32K was reacted with beta-amanitin hydroxysuccinimide ester to form an amide-linked toxin conjugate. Loading of the polymeric chain with amanitin was high, corresponding to up to 35% of the total weight. To this amatoxin vehicle we attached a targeting molecule, human recombinant leucine-21 epidermal growth factor (hrEGFL), via a disulfide-containing linker moiety. A typical average stoichiometry of the hrEGFL labeled toxin conjugate was (L-Orn)164(beta-amanitin)19(COC2H4SSC2H4CO-hrEGFL)2. The affinity for EGF receptors of hrEGFL bound in this conjugate was tested by using A 431 cells. The affinity was eight times lower than that of unsubstituted hrEGFL but regarded as high enough for studying specific toxicity effects with cells bearing EGF receptors. We found that beta-amanitin in the labeled conjugate was able to inhibit the growth of A 431 cells at a concentration of 28 nM, 80 times lower than for native beta-amanitin and 20 times lower than for poly-L-ornithine-bound beta-amanitin without the hrEGFL label. The approximately 20-fold enhancement of cytotoxicity suggests a specific internalization of the toxin conjugate mediated by the hormone label. This idea is supported by the fact that also in another transformed fibroblast cell line, with an increased though smaller number of EGF receptors than A 431 cells, the corresponding enhancement of cytotoxicity was demonstrable but less pronounced (7-fold). The hormone-mediated increase in cytotoxicity of EGF labeled poly-L-ornithine-beta-amanitin conjugates, combined with their moderate toxicity in the mouse, encourages further examination of such compounds in tumor model systems in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Toward regulation of gonadal function by a synthetic hybrid molecule composed of gonadotropin and Fc fragment of immunoglobulin G

A conjugate of human chorionic gonadotropin (HCG) and Fc fragment of immunoglobulin G was prepared by covalent cross-linking using the heterobifunctional reagent, N-succinimidyl 3-(2-pyridyldithio) propionate. Mouse Leydig tumor cells expressing receptors for luteinizing hormone were specifically lysed in vitro as a consequence of complement fixation via the Fc component of the hybrid molecule. Furthermore, administration of HCG-Fc to rams caused an acute depression in circulatory testosterone. This novel concept of targeted inhibition of gonadal function could prove to have future applications in control of reproductive processes.

Hormonotoxins: conjugation of human choriogonadotropin with the ribosome inactivating protein gelonin and comparison with a lutropin conjugate

The properties of two hormonotoxins, human choriogonadotropin and ovine pituitary lutropin, each conjugated to the ribosome inactivating protein gelonin, have been compared. In both cases the linkage is effected through a disulfide bond provided by the cross-linker N-succinimidyl-3-(2-pyridyldithio)propionate. In each conjugate the hormone and toxin were calculated to be present in a ratio of 1:1. Immunological reactivity, receptor binding activity and steroidogenic activity of the hormonotoxins in radioimmunoassay or in membranes or cells containing LH receptors were reduced but not abolished in comparison with the thiolated hormones. Purified hormonotoxins were characterized by immunoassays specific for both interaction components, viz. the hormone (LH/hCG) and gelonin. Binding of the toxic component (gelonin) to the mouse Leydig tumour cells (MA-10 cells) was shown to occur via the hormone part of the hormonotoxins. This was effectively competed by the native hormone but not free gelonin. Both hormonotoxins were cytotoxic to MA-10 cells as protein synthesis was inhibited in their presence. This was associated with destruction of tumor cells in culture.

A superactive hormonotoxin prepared with truncated diphtheria toxin

A chemically truncated form of diphtheria toxin, DT51, which lacks the cell-binding site but retains the membrane-translocating function, was covalently linked to luteinizing hormone (LH) and compared to similar conjugates containing diphtheria toxin (DT) or diphtheria toxin A-chain (DTA). The DT51 hormonotoxin killed cells possessing an LH receptor at concentrations similar to that of DT hormonotoxin and orders of magnitude lower than DTA hormonotoxin. The DTA hormonotoxin exhibited an LD-50 similar to that of previously reported hormonotoxins which employed DTA, ricin A-chain, or gelonin as toxic moieties.