Synthesis of methotrexate-antibody conjugates by regiospecific coupling and assessment of drug and antitumor activities

Antibody drug conjugates beyond cytotoxic payloads

For many years, antibody drug conjugates (ADC) have teased with the promise of targeted payload delivery to diseased cells, embracing the targeting of the antibody to which a cytotoxic payload is conjugated. During the past decade this promise has started to be realised with the approval of more than a dozen ADCs for the treatment of various cancers. Of these ADCs, brentuximab vedotin really laid the foundations of a template for a successful ADC with lysosomal payload release from a cleavable dipeptide linker, measured DAR by conjugation to the Cys-Cys interchain bonds of the antibody and a cytotoxic payload. Using this ADC design model oncology has now expanded their repertoire of payloads to include non-cytotoxic compounds. These new payload classes have their origins in prior medicinal chemistry programmes aiming to design selective oral small molecule drugs. While this may not have been achieved, the resulting compounds provide excellent starting points for ADC programmes with some compounds amenable to immediate linker attachment while for others extensive SAR and structural information offer invaluable design insights. Many of these new oncology payload classes are of interest to other therapeutic areas facilitating rapid access to drug-linkers for exploration as non-oncology ADCs. Other therapeutic areas have also pursued unique payload classes with glucocorticoid receptor modulators (GRM) being the most clinically advanced in immunology. Here, ADC payloads come full circle, as oncology is now investigating GRM payloads for the treatment of cancer. This chapter aims to cover all these new ADC approaches while describing the medicinal chemistry origins of the new non-cytotoxic payloads.

The role of Cannabidiol and tetrahydrocannabivarin to overcome doxorubicin resistance in MDA-MB-231 xenografts in athymic nude mice

The significant resistance to currently available chemotherapeutics makes treatment for TNBC a key clinical concern. Herein, we studied the anti-cancer potentials of synthetic cannabidiol (CBD) and Tetrahydrocannabivarin (THCV) when used alone or in combination with doxorubicin (DOX) against MDA-MB-231 resistant cells. Pre-treatment with CBD and THCV significantly increased the cytotoxicity of DOX in MDA-MB-231 2D and 3D cultures that were DOX-resistant. Transcriptomics and Proteomics studies revealed that CBD and THCV, by downregulating PD-L1, TGF-β, sp1, NLRP3, P38-MAPK, and upregulating AMPK induced apoptosis leading to improved DOX's chemosensitivity against DOX resistant MDA-MB-231 tumors in BALB/c nude mice. CBD/THCV in combination with DOX significantly inhibited H3k4 methylation and H2K5 acetylation as demonstrated by western blotting and RT-PCR. Based on these findings, CBD and THCV appear to counteract histone modifications and their subsequent effects on DOX, resulting in chemo-sensitization against MDA-MB-231 resistant cancers.

Breast cancer: insights in disease and influence of drug methotrexate

According to the World Health Organization, cancer is one of the leading causes of morbidity and mortality worldwide. The previously estimated 14 million new cases in the year of 2012 are expected to rise, yearly, over the following 2 decades. Among women, breast cancer is the most common one. In 2012, almost 1.7 million people were diagnosed worldwide and half a million died from the disease. Despite having several treatments available, from surgery to chemotherapy, most of these treatments have severe adverse effects. Chemotherapy has a narrow therapeutic window and requires high dosage treatment in patients with advanced-stage cancers and further need innovative treatment strategies. Although methotrexate (MTX) is not a first line drug used against breast cancer, however, it might be valuable to fight the disease. MTX is an effective and cheap drug that might impair malignant growth without irreversible damage to normal tissues. Nevertheless, while MTX does present some disadvantages including poor solubility and low permeability, several strategies are being used to discover and provide novel and effective targeted treatment against breast cancer. In this review, we analyze the chemotherapy of breast cancer and its relationship with drug MTX.

Synthesis and Evaluation of Hydrogen Peroxide Sensitive Prodrugs of Methotrexate and Aminopterin for the Treatment of Rheumatoid Arthritis

A series of novel hydrogen peroxide sensitive prodrugs of methotrexate (MTX) and aminopterin (AMT) were synthesized and evaluated for therapeutic efficacy in mice with collagen induced arthritis (CIA) as a model of chronic rheumatoid arthritis (RA). The prodrug strategy selected is based on ROS-labile 4-methylphenylboronic acid promoieties linked to the drugs via a carbamate linkage or a direct C-N bond. Activation under pathophysiological concentrations of H₂O₂ proved to be effective, and prodrug candidates were selected in agreement with relevant in vitro physicochemical and pharmacokinetic assays. Selected candidates showed moderate to good solubility, high chemical and enzymatic stability, and therapeutic efficacy comparable to the parent drugs in the CIA model. Importantly, the prodrugs displayed the expected safer toxicity profile and increased therapeutic window compared to MTX and AMT while maintaining a comparable therapeutic efficacy, which is highly encouraging for future use in RA patients.

A sequential enzyme-activated and light-triggered pro-prodrug nanosystem for cancer detection and therapy

A sequential enzyme-activated and light-triggered pro-prodrug has been developed for cancer biomarker detection and on-demand therapy.

Evaluation of small ligand-protein interactions by using T7 RNA polymerase with DNA-modified ligand

The interaction between proteins and ligands was evaluated by T7 RNA polymerase transcription with a DNA-modified ligand. The principle of this method is suppression of T7 RNA polymerase transcription by binding of a protein to small ligand modified by conjugation with a T7 RNA polymerase promoter. To demonstrate proof of principle, biotin or antifolate methotrexate was modified by covalent attachment of a T7 RNA promoter. Using these T7 RNA promoter-modified ligands, T7 RNA polymerase transcriptions were performed in the presence or absence of an anti-biotin antibody or recombinant human dihydrofolate reductase, respectively. Transcription was suppressed in the presence of each binding protein plus its modified ligand, but not in the absence of the binding protein.

A quantitative assessment of nanoparticle-ligand distributions: implications for targeted drug and imaging delivery in dendrimer conjugates

Functional nanoparticles often contain ligands including targeting molecules, fluorophores, and/or active moieties such as drugs. Characterizing the number of these ligands bound to each particle and the distribution of nanoparticle-ligand species is important for understanding the nanomaterial's function. In this study, the amide coupling methods commonly used to conjugate ligands to poly(amidoamine) (PAMAM) dendrimers were examined. A skewed Poisson distribution was observed and quantified using HPLC for two sets of dendrimer-ligand samples prepared using the amine-terminated form of the PAMAM dendrimer and a partially acetylated form of the PAMAM dendrimer that has been used for targeted in vivo drug delivery. The prepared samples had an average number of ligands per dendrimer ranging from 0.4 to 13. Distributions identified by HPLC are in excellent agreement with the mean ligand/dendrimer ratio, measured by (1)H NMR, gel permeation chromatography (GPC), and potentiometric titration. These results provide insight into the heterogeneity of distributions that are obtained for many classes of nanomaterials to which ligands are conjugated and belie the use of simple cartoon models that present the "average" number of ligands bound as a physically meaningful representation for the material.

Inhibition studies of sulfonamide-containing folate analogs in yeast

In the folate biosynthetic pathway, sulfa drugs (sulfonamides and sulfones) compete with the natural substrate, para-aminobenzoate (pABA) causing depletion of dihydrofolate (DHF) and subsequent growth inhibition. The sulfa drugs condense with 2-amino-4-hydroxy-6-hydroxymethyl-7,8 dihydropteridine pyrophosphate (DHPPP) forming sulfa-dihydropteroate (sulfa-DHP). Here evidence is presented using yeast that such dihydropteroate (DHP) analogs are inhibitory through competition with DHF. Two folate synthesis mutants, with respective dihydrofolate synthase (DHFS) and dihydropteroate synthase (DHPS) deletions and requiring DHF for growth were exposed to sulfa drugs. The DHFS knockout mutant was inhibited, but the DHPS knockout mutant that was incapable of forming sulfa-DHP was insensitive. Such sulfa-DHP compounds were chemically synthesized and shown to be inhibitory in vivo by competing with DHF, but in vitro assays with double the concentration of the sulfa-DHP to DHF showed no inhibition of dihydrofolate reductase (DHFR). Sequence analysis of resistant mutants obtained in the presence of sulfa drugs showed no changes in DHFR, or DHPS, unlike previously found antifolate-resistant mutants. The diamino derivatives, which are precursors of the sulfa-DHP, were found to be DHFR inhibitors. These results suggest that a new class of drugs, based on DHP analogs, could be investigated.

Inhalation delivery of anticancer agents via HFA-based metered dose inhaler using methotrexate as a model drug

In the present study, the feasibility of delivering anticancer drugs via metered dose inhaler (MDI) was demonstrated using methotrexate (MTX) as a model anticancer drug. MDI formulations of MTX were prepared using hydrofluoroalkane-134a containing 0.67% MTX and 10% ethyl alcohol. The particle size of MTX was reduced by cryo milling with or without a surfactant (Pluronic F77) and the milled drug was employed for MDI formulations, which were subsequently evaluated for their medication delivery, mass median aerodynamic diameter (MMAD) and geometric standard deviation (GSD). Further, the efficacy of aerosolized MTX was evaluated by determining the in-vitro cytotoxicity of MTX against HL-60 cells using a six-stage viable impactor and the induction of apoptosis in HL-60 cells by acridine orange staining. Our results indicate that MTX aerosols having an MMAD varying between 2.2 and 3.2 microm (GSD 2.6-3.7) with a respirable fraction varying between 14.2 and 17.1% could be obtained by using MTX, which was cryo milled either alone or in combination with Pluronic F77. Exposure of HL-60 cells plated in third, fourth, fifth, and sixth stages of viable impactor to two actuations of MDI showed a cell kill of greater than 50%. Further, aerosolized MTX was found to induce apoptosis in HL-60 cells, as assessed by the morphological examination of the cells with fluorescent and confocal microscopy. Our results demonstrate that it is possible to deliver cytotoxic concentrations of MTX in an in vitro system simulating the lower respiratory tract (by using a six-stage viable impactor) via MDI and the cytotoxicity of the aerosolized MTX could be further improved by the optimization of the aerodynamic size.

Targeted drug conjugates: principles and progress

Reports of targeting drugs using antibodies have appeared in the literature since 1958, but exciting clinical results in this field have only been reported in the last few years. Progress in this field has occurred largely through an understanding how drug-immunoconjugates work. The objective of this review is to draw together the fundamental principles on which this field of work is based, to examine the evidence supporting those principles, and the effectiveness and selectivity of targeted drug conjugates. The activity of many drug-immunoconjugates can now largely be accounted for by the underlying principles. Excellent development work, both with conventional anti-cancer agents and very potent drugs have led to a number of interesting clinical trials. In the best Phase I and II trials, good evidence of effectiveness have been reported, which suggest that drug-immunoconjugates may now be heralding a new era for chemotherapy.

Phosphinic acid pseudopeptides analogous to glutamyl-gamma-glutamate: synthesis and coupling to pteroyl azides leads to potent inhibitors of folylpoly-gamma-glutamate synthetase

Several routes to a complex phosphinate phosphapeptide analogous to the gamma-glutamyl peptide Glu-gamma-Glu have been investigated. Formation of gamma-phosphono glutamate derivatives via addition of a phosphorus-based radical to protected vinylglycine was found to be of limited value because of the elevated temperatures required. Alkylation and conjugate addition reactions of trivalent phosphorus (P(III)) species were investigated. In situ generation of bis-trimethylsilyl esters of phosphinous acids proved to be an effective route to phosphinates of modest structural complexity. However, this chemistry could not be extended to the incorporation of an amino acid moiety at the N-terminal side of the desired phosphinate. A successful synthesis of the target phosphinate phosphapeptide was effected using P(III) chemistry and dehydrohalogenation to yield an alpha,beta-unsaturated phosphinic acid ester, following which conjugate addition of diethylacetamido malonate and acid-mediated hydrolysis afforded the desired phosphinate phosphapeptide. Coupling of the unprotected phosphinate phosphapeptide with two acyl azides derived from folic acid and methotrexate led to the corresponding pteroylphosphapeptides of interest as possible mimics of tetrahedral intermediates in the reaction catalyzed by folylpolyglutamate synthetase.

Activation of methotrexate-phenylalanine by monoclonal antibody--carboxypeptidase A conjugate for the specific treatment of ovarian cancer in vitro

Monoclonal antibody 4E3 directed against a glycosylated surface protein on human ovarian teratocarcinoma cells (CRL-1572 cell line) was conjugated to bovine carboxypeptidase A (CPA) using a 3400 Da polyethylene glycol chain bearing an N-hydroxysuccinimide group at both ends. The conjugate preparation was purified by fast protein liquid chromatography on a Superose 12/30 HR column. The 4E3-CPA conjugate was recovered in the third fraction by SDS-PAGE analysis. The specific binding of the 4E3-CPA conjugate to CRL-1572 cells was confirmed by a FACS analysis and the enzymatic activity of the conjugate remained while tested with hippuryl-L-phenylalanine. In vitro cytotoxic assays on CRL-1572 cells showed that the prodrug methotrexate-phenylalanine (MTX-Phe) alone was non-toxic (ID50 > 1000 ng ml-1) but was selectively converted to MTX when the cells were pretreated with 50 micrograms ml-1 4E3-CPA conjugate, which enhanced considerably the pharmacological activity of the prodrug with an ID50 of 70 ng ml-1. The co-culture assays with CRL-1572 and MRC-5 cells (human normal lung diploid fibroblast cell lines) demonstrated the specificity of the 4E3-CPA conjugate for the CRL-1572 cells since no cytotoxicity was observed on the MRC-5 cells. When both cell lines were mixed in ratios ranging from 1:10,000 to 1:5 (CRL-1572:MRC-5), the significant increase in the ID25 was correlated with the proportion of tumoral cells present in the cell inoculum. These results suggest that MTX-Phe combined with 4E3-CPA conjugate is a promising model for a more selective and localised anti-cancer chemotherapy based on the ADEPT concept.

A new method of technetium-99m labeling of monoclonal antibodies through sugar residues. A study with TAG-72 specific CC-49 antibody

We have developed a very efficient labeling technique for monoclonal antibodies with technetium-99m. Oxidation of sugar residues on the IgG class of antibodies leads to the generation of aldehyde groups which are further reacted with two newly developed hydrazide compounds. This methodology introduces sulfhydryl groups on the antibody through sugar residues which can be labeled with technetium-99m. We have studied the TAG-72 specific second generation antibody CC-49. The specific activity of the labeled antibody was high without loss of its immunoreactivity.

Selective coupling of methotrexate to peptide hormone carriers through a gamma-carboxamide linkage of its glutamic acid moiety: benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophosphate activation in salt coupling

A convenient synthetic method is described for the preparation of peptide-methotrexate (MTX) conjugates in which MTX is coupled selectively through the gamma-carboxyl group of its glutamic acid moiety to a free amino group in peptide analogs. The syntheses of a somatostatin analog-MTX conjugate (MTX-D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Thr-NH2) (AN-51) and two conjugates of analogs of luteinizing hormone-releasing hormone (LH-RH) with MTX [Glp-His-Trp-Ser-Tyr-D-Lys(MTX)-Leu-Arg-Pro-Gly-NH2] (AJ-04) and [Ac-Ser-Tyr-D-Lys(MTX)-Leu-Arg-Pro-NH-Et] AJ-51 are presented as examples. Benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophosphate (BOP reagent) was used in the synthesis for activation of 4-amino-4-deoxy-N10-methylpteroic acid, which reacted with the potassium salt of glutamic acid alpha-tert-butyl ester in dimethyl sulfoxide to form the suitably protected MTX derivative. This synthesis provides an example of the high suitability of BOP reagent for the salt-coupling method. The selectively protected MTX derivative was then coupled to the different peptide carriers and deprotected under relatively mild conditions by trifluoroacetic acid. The conjugates of MTX with hormonal analogs are suitable for targeting to various tumors that possess receptors for the peptide moieties.

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.

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.

Tissue localization of methotrexate-monoclonal-IgM immunoconjugates: anti-SSEA-1 and MOPC 104E in mouse teratocarcinomas and normal tissues

Methotrexate (MTX) was coupled to the tumor-targeting monoclonal IgM, anti-SSEA-1 and the non-targeting myeloma IgM, MOPC 104E. At 24-h intervals following injection, drug deposition in MH-15 teratocarcinomas and in several normal tissues was followed by immunoperoxidase microscopy using the M16 monoclonal antibody to MTX. MTX-anti-SSEA-1 was deposited on the surface and in the interior of living tumor cells 24 h after injection; at 48 h and after, only low-level binding to necrotic tissue was found. There was no significant gradation in staining from the outside to the interior of the tumors. In tumors, the control MOPC 104E immunoconjugate was detectable only in necrotic tissue. Binding to SSEA-1-expressing normal tissues was undetectable, except for pericryptal fibroblasts in the small intestine. No significant pathology was found in normal tissues that are SSEA-1 positive. High levels of the immunoconjugate were detected in the liver, where MTX was found predominantly in Kupffer cells and possibly in hepatocytes; again, no significant morphological changes were associated with this retention. Thus tumor-associated antigens can be suitable targets for antibody-drug conjugates even when present in normal tissues and in large quantities, provided that the antigens in normal tissues are inaccessible. Moreover, deposition in viable tumor tissue can be assessed using monoclonal antibodies to methotrexate.

The influence of periodate oxidation on monoclonal antibody avidity and immunoreactivity

Eight monoclonal antibodies of different classes and isotypes and rabbit IgG were oxidized under a variety of conditions with 5-50 mM periodate. The number of aldehyde groups generated per immunoglobulin were measured by reduction with tritiated sodium borohydride or coupling of fluoresceinthiosemicarbazide. There were up to 25.5 aldehyde groups detected on the periodate-oxidized antibody 96.5, measured by borohydride reduction whereas the same conditions led to only 9.6 aldehydes per IgG on the antibody L6 of the same IgG2A isotype. Fluoresceinthiosemicarbazide bound to oxidized antibodies but not to the same extent as tritium. On mildly oxidized IgMs it was possible to generate more than 200 aldehyde groups per antibody molecule. Depending on the conditions and the antibody used periodate oxidation could lead to antibody crosslinking. The avidities of the modified antibodies were determined by Scatchard analyses and inhibition assays. A new mathematical method to evaluate the immunoreactivities of modified antibodies relative to the unlabeled native antibody from inhibition binding data was established. Periodate concentrations higher than 50 mM decreased the avidities and immunoreactivities of all IgGs tested. This effect is more pronounced if the oxidation is performed at pH 5.6 and 25 degrees C instead of pH 4.6 and 0 degree C. The BR96 antibody is inactivated even under mild oxidation conditions.

Anti-transferrin receptor antibody and antibody-drug conjugates cross the blood-brain barrier

Delivery of nonlipophilic drugs to the brain is hindered by the tightly apposed capillary endothelial cells that make up the blood-brain barrier. We have examined the ability of a monoclonal antibody (OX-26), which recognizes the rat transferrin receptor, to function as a carrier for the delivery of drugs across the blood-brain barrier. This antibody, which was previously shown to bind preferentially to capillary endothelial cells in the brain after intravenous administration (Jefferies, W. A., Brandon, M. R., Hunt, S. V., Williams, A. F., Gatter, K. C. & Mason, D. Y. (1984) Nature (London) 312, 162-163), labels the entire cerebrovascular bed in a dose-dependent manner. The initially uniform labeling of brain capillaries becomes extremely punctate approximately 4 hr after injection, suggesting a time-dependent sequestering of the antibody. Capillary-depletion experiments, in which the brain is separated into capillary and parenchymal fractions, show a time-dependent migration of radiolabeled antibody from the capillaries into the brain parenchyma, which is consistent with the transcytosis of compounds across the blood-brain barrier. Antibody-methotrexate conjugates were tested in vivo to assess the carrier ability of this antibody. Immunohistochemical staining for either component of an OX-26-methotrexate conjugate revealed patterns of cerebrovascular labeling identical to those observed with the unaltered antibody. Accumulation of radiolabeled methotrexate in the brain parenchyma is greatly enhanced when the drug is conjugated to OX-26.

Regression of human melanoma xenografts in nude mice injected with methotrexate linked to monoclonal antibody 225.28 to human high molecular weight-melanoma associated antigen

Intravenous injections into nude mice of 5 mg/kg methotrexate (MTX) linked to the antibody to human high molecular weight-melanoma associated antigen (HMW-MAA), monoclonal antibody (mAb) 225.28, an IgG2a, on days 1, 4, 7, 10 and 14, starting 24 h after subcutaneous inoculation of 2 x 10(6) cultured human M21 melanoma cells inhibited mean tumor volume by 90% on day 14 and by 65% on day 50 after the beginning of the treatment. Injections of equimolar amounts of free MTX and MTX linked to normal mouse IgG or to an isotype-matched myeloma protein did not inhibit tumor growth significantly. MTX linked to mAb 225.28 did not inhibit the xenograft of a subline of human melanoma cell line M21 without detectable expression of HMW-MAA. In a clonogenic assay, the MTX-225.28 conjugate was three times more potent in inhibiting the growth of M21 melanoma cells than free MTX, but did not inhibit the growth of kidney carcinoma cells Caki-1, which do not express high-Mr MAA. In contrast, MTX linked to the mAb DAL K29, reacting with kidney carcinoma cells Caki-1, inhibited their growth but did not affect that of melanoma cells. M21 melanoma cells isolated from the residual tumor of a mouse treated with the MTX-225.28 conjugate did not differ in their reactivity with mAb 225.28 and in their sensitivity to MTX when compared with M21 cells from an untreated mouse.