Intratumoral conversion of 5-fluorocytosine to 5-fluorouracil by monoclonal antibody-cytosine deaminase conjugates: noninvasive detection of prodrug activation by magnetic resonance spectroscopy and spectroscopic imaging.

The monitoring of antibody-directed enzyme-prodrug therapies requires evaluation of drug activation within the tissues of interest. We have demonstrated the feasibility of noninvasive magnetic resonance spectroscopy and spectroscopic imaging (chemical shift imaging) to detect activation of the prodrug 5-fluorocytosine (5-FCyt) to the cytotoxic species 5-fluorouracil (5-FU) by monoclonal antibody-cytosine deaminase (CD) conjugates. In vitro, L6-CD but not 1F5-CD selectively metabolized 5-FCyt to 5-FU on H2981 human lung adenocarcinoma cells because of the presence and absence of cell surface L6 and CD20 antigens, respectively. After pretreatment of H2981 tumor-bearing mice with L6-CD, in vivo metabolism of 5-FCyt to 5-FU within the tumors was detected by 19F magnetic resonance spectroscopy; the chemical shift separation between 5-FCyt and 5-FU resonances was approximately 1.2 ppm. 5-FU levels were 50-100% of 5-FCyt levels in tumors 10-60 min after 5-FCyt administration. Whole body 19F chemical shift imaging (6 x 6 mm in-plane resolution) of tumor-bearing mice demonstrated the highest signal intensity of 5-FU within the tumor region. This study supports further development of noninvasive magnetic resonance methods for preclinical and clinical monitoring of CD enzyme-prodrug therapies.

N-(4'-hydroxyphenylacetyl)palytoxin: a palytoxin prodrug that can be activated by a monoclonal antibody-penicillin G amidase conjugate.

Palytoxin (PTX), one of the most toxic nonprotein molecules known, is cytotoxic at picomolar concentrations against a wide variety of cell types. In contrast to most cytotoxins, PTX exerts its activity extracellularly. A method for targeting PTX to tumor cells is described in which a monoclonal antibody-enzyme conjugate activates a PTX prodrug at surfaces of tumor cells. The prodrug, N-(4'-hydroxyphenylacetyl)palytoxin (NHPAP), was prepared by reacting PTX with an active ester of 4-hydroxyphenylacetic acid. NHPAP was 1000 times less toxic than PTX to a panel of carcinoma and lymphoma cell lines. The cytotoxic activity of the combination of penicillin G amidase from Escherichia coli with NHPAP was equal to PTX. Two cell lines that were multidrug resistant showed no enhanced resistance to NHPAP +/- penicillin G amidase. Immunologically specific activation of NHPAP took place when H2981 cells (L6 antigen positive) were treated with the monoclonal antibody conjugate L6-penicillin G amidase followed by NHPAP. This system is distinguished from other prodrug activation schemes, since the released drug exerts its activity extracellularly, has high potency, and may be able to overcome the multidrug resistant phenotype.

Enhancement of the in vitro and in vivo antitumor activities of phosphorylated mitomycin C and etoposide derivatives by monoclonal antibody-alkaline phosphatase conjugates.

Alkaline phosphatase (AP) was covalently linked to the two antitumor monoclonal antibodies, L6 (anticarcinoma) and 1F5 (anti-B lymphoma), forming conjugates that could bind to antigen-positive tumor cells. The conjugates were able to convert the prodrugs, mitomycin phosphate (MOP) and etoposide phosphate (EP), into an active mitomycin C derivative, mitomycin alcohol, and etoposide, respectively. MOP and EP were less toxic to cultured cells from the H2981 lung adenocarcinoma than their respective hydrolysis products, mitomycin alcohol and etoposide, by a factor greater than 100, and they were also less toxic in mice. Pretreatment of H2981 cells with L6-AP greatly enhanced the cytotoxic effects of MOP and EP, while 1F5-AP caused no such enhancement. A strong antitumor response was observed in H2981-bearing mice that were treated with L6-AP followed 24 h later by either MOP or a combination of MOP and EP. This response was superior to that of MOP or combinations of MOP and EP given alone.

The role of rat serum carboxylesterase in the activation of paclitaxel and camptothecin prodrugs.

Paclitaxel-2-ethylcarbonate (PC) is a prototype for a family of paclitaxel prodrugs that have significant levels of antitumor activities in rodent models for human cancer. In this study, an enzyme responsible for the conversion of PC to paclitaxel was purified from rat serum. N-terminal amino acid sequence analysis indicated that the isolated enzyme was rat serum carboxylesterase. This enzyme was shown to significantly enhance the cytotoxic activities of both PC and 7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin (CPT-11), a water-soluble analogue of camptothecin, on lung carcinoma and melanoma cell lines. Rat serum carboxylesterase may have applications for the site-specific delivery of anticancer drugs to tumor masses.

Intratumoral generation of 5-fluorouracil mediated by an antibody-cytosine deaminase conjugate in combination with 5-fluorocytosine.

Cytosine deaminase (CD) is a microbial enzyme that can convert the antifungal agent 5-fluorocytosine (5-FC) into the antitumor agent, 5-fluorouracil (5-FU). The enzyme was chemically conjugated to the L6 monoclonal antibody, forming a conjugate that bound to antigens on the H2981 lung adenocarcinoma. Detailed studies were undertaken to determine the extent to which L6-CD generated 5-FU in tumor-bearing mice. Very high tumor:blood ratios of L6-CD (42:1) in vivo were obtained by injecting the conjugate followed 24 h later by an antiidiotypic antibody that could bind to circulating L6-CD but not to L6-CD that was bound to H2981 cells. As a result, significantly more 5-FC could be administered (> 800 mg/kg) than 5-FU (90 mg/kg). L6-CD converted 5-FC into 5-FU such that the L6-CD/antiidiotypic monoclonal antibody/5-FC combination resulted in 17 times more intratumoral 5-FU compared to systemic 5-FU administration. The conversion was antigen dependent since much lower intratumoral 5-FU levels were obtained in H3719 tumors that failed to localize L6-CD. The conversion of 5-FC into 5-FU was low in blood, kidneys, and liver. This demonstrates that a major increase in intratumoral drug concentrations can be attained with an monoclonal antibody-enzyme conjugate in combination with an anticancer prodrug compared to systemic drug therapy.

Usefulness of repeated direct intratumoral gene transfer using hemagglutinating virus of Japan-liposome method for cytosine deaminase suicide gene therapy.

To investigate the feasibility of repeated gene transfection in suicide gene therapy against human solid tumors by a combination of 5- fluorocytosine (5-FC) and its converting enzyme, cytosine deaminase (CD), we repeatedly transfected the yeast CD gene into the human pancreatic cancer cell line BXPC3 using the hemagglutinating virus of Japan-liposome in a new gene transfer method. The in vivo growth of the s.c. transplanted BXPC3 tumor in nude mice given CD-gene transfection was significantly suppressed by i.p. injection of 5-FC when compared with tumors treated with the control vector. Furthermore, the tumor transfected with the CD gene during a 7-day interval was suppressed much more than that of a single transfection. These results suggest that repeated transfection of the suicide gene together with the combination of 5-FC and the yeast CD gene using the hemagglutinating virus of Japan-liposome gene transfer method may be useful for the treatment of human solid tumors, including pancreatic cancer.

Regressions and cures of melanoma xenografts following treatment with monoclonal antibody beta-lactamase conjugates in combination with anticancer prodrugs.

Cephalosporin doxorubicin (C-Dox) and 7-(4-carboxybutanamido)-cephalosporin mustard (CCM) are prodrugs that are catalytically converted by Enterobacter cloacae beta-lactamase (bL) to the active anticancer agents doxorubicin and phenylenediamine mustard, respectively. Both prodrugs were less cytotoxic to the 3677 human melanoma line than their respective drugs and were activated in an immunologically specific manner by 96.5-bL, a mAb-bL conjugate that binds to 3677 cell surface antigens. Similar results were obtained using the CCM prodrug on SK-MEL 28 human melanoma cells. Experiments in mice with established s.c. 3677 tumors demonstrated that although no tumors were cured in mice receiving the 96.5-bL/C-Dox combination, the activities were greater than those obtained from systemic doxorubicin treatment or from administration of the nonbinding conjugate P1.17-bL in combination with C-Dox. In contrast, when CCM was used as a prodrug, cures of established 3677 tumors were obtained in 80% of the 96.5-bL treated animals. This combination was also able to induce regressions of large 3677 tumor masses (800 mm3) without any apparent toxic side effects. We conclude that 96.5-bL in combination with C-Dox or CCM has greater antitumor activity than systemic treatment with the corresponding drugs and that CCM is a more effective prodrug than C-Dox for treating human 3677 melanoma xenografts.

In vitro and in vivo activities of a doxorubicin prodrug in combination with monoclonal antibody beta-lactamase conjugates.

A cephalosporin derivative of doxorubicin (C-Dox) was evaluated as a prodrug for activation by mAb conjugates of the beta-lactamase from Enterobacter cloacae P99 (beta L; EC 3.5.2.6). The conjugates consisted of beta L and the F(ab') fragments of either of the mAbs L6, P1.17, or 96.5. L6 binds to antigens on a variety of carcinomas, including the two lung adenocarcinoma cell lines H2981 and H2987 used in this study. 96.5 binds to the melanoma-associated antigen p97, and P1.17 was used for the control conjugate. C-Dox was found to be less cytotoxic to three different tumor cell lines in vitro compared to the parent drug doxorubicin (Dox). Immunospecific activation took place when the cells were pretreated with beta L conjugates that could bind to antigens on the tumor cells. In vivo toxicity and pharmacokinetics studies in athymic female nu/nu mice revealed that C-Dox was at least 7-fold less toxic than Dox (on a molar basis), despite the fact that a > or = 320-fold greater area-under-the-curve (blood concentration versus time) of C-Dox compared to Dox was obtained 0-2 h after administration of the two agents. Pharmacokinetic studies at maximum tolerated doses in mice bearing xenografts of either H2981 or H2987 revealed that the intratumoral levels of Dox after treatment with L6-beta L in combination with C-Dox were higher than were obtained by either systemic treatment with Dox or a combination of P1.17-beta L and C-Dox. This finding suggested that the conversion of C-Dox to Dox was tumor specific and dependent on the presence of the targeted antigen. Furthermore, the best antitumor activity against both H2981 and H2987 tumors was obtained by treatment with L6-beta L and C-Dox compared to P1.17-beta L and C-Dox or Dox alone. Thus, higher levels of Dox corresponded to greater therapeutic effects in both of the tumor models studied.

Genetic construction, expression, and characterization of a single chain anti-carcinoma antibody fused to beta-lactamase.

We report the genetic construction and expression of a fusion protein between an antibody single chain-linked variable domain fragment specific for human carcinomas and beta-lactamase II from Bacillus cereus. Sequences encoding the variable regions of the L6 monoclonal antibody were assembled so as to be separated from each other by an 18-amino acid linker and from the mature form of beta-lactamase by a 6-amino acid linker. The construct was placed under the transcriptional regulation of the lac promoter, and the PelB signal sequence was used to direct export of the fusion protein to the periplasmic space of Escherichia coli. After induction, biologically active material was recovered from both culture supernatants and cell lysates. Affinity chromatography yielded about 2.5 micrograms of protein/ml of initial culture volume. The fusion protein was shown to bind to tumor cells at least as well as chemically prepared F(ab') and to maintain beta-lactamase activity at a level similar to that of the native enzyme. Tumor cells coated with the fusion protein were sensitive to a cephalosporin mustard prodrug in a dose-dependent fashion comparable to that of enzyme chemically conjugated to F(ab'). This article demonstrates the feasibility of using single chain-linked variable domain-enzyme fusion proteins for the activation of anticancer prodrugs.

Selective activation of anticancer prodrugs by monoclonal antibody-enzyme conjugates.

A great deal of interest has surrounded the activities of monoclonal antibodies (mAbs), and mAb-drug, toxin and radionuclide conjugates for the treatment of human cancers. In the last few years, a number of new mAb-based reagents have been clinically approved (Rituxan, Herceptin, and Panorex), and several others are now in advanced clinical trials. Successful therapeutic treatment of solid tumors with drug conjugates of such macromolecules must overcome the barriers to penetration within tumor masses, antigen heterogeneity, conjugated drug potency, and efficient drug release from the mAbs inside tumor cells. An alternative strategy for drug delivery involves a two-step approach to cancer therapy in which mAbs are used to localize enzymes to tumor cell surface antigens. Once the conjugate binds to the cancer cells and clears from the systemic circulation, antitumor prodrugs are administered that are catalytically converted to active drugs by the targeted enzyme. The drugs thus released are capable of penetrating within the tumor mass and eliminating both cells that have and have not bound the mAb-enzyme conjugate. Significant therapeutic effects have been obtained using a broad range of enzymes along with prodrugs that are derived from both approved anticancer drugs and highly potent experimental agents. This review focuses on the activities of several mAb-enzyme/prodrug combinations, with an emphasis on those that have provided mechanistic insight, clinical activity, novel protein constructs, and the potential for reduced immunogenicity.

Immunologically specific activation of a cephalosporin derivative of mitomycin C by monoclonal antibody beta-lactamase conjugates.

The syntheses of two cephalosporin derivatives 2 and 3 of mitomycin C (1) containing 7-phenylacetamido and 7-delta-carboxybutanamido side chains, respectively, are described. These compounds were prepared for evaluation as cephalosporin prodrugs capable of being activated by mAb-beta-lactamase conjugates. In vitro cytotoxicity assays performed on H2987 lung adenocarcinoma and clone 62 melanoma cell lines indicated that compound 2 was comparable in cytotoxicity to the parent drug. In an effort to improve upon the cytotoxic differential of 2, an alternative prodrug 3 containing a polar carboxyl group in the side chain of the cephalosporin moiety was prepared. Compound 3 consistently behaved as a prodrug and was approximately 40- and 10-fold less toxic than 1 toward H2987 and clone 62, respectively. Determination of kinetic constants for hydrolysis by beta-lactamase from Enterobacter cloacae P99 indicated kcat values of 476 +/- 170 and 248 +/- 15.1 s-1 for 2 and 3, respectively. The kcat/Km ratios for 2 and 3 were found to be approximately 9.7 and 2.1 microM/s, respectively. Comparison of these kcat/Km values with those obtained for similar cephalosporin derivatives of other antitumor agents demonstrated that compounds with delta-carboxybutanamido side chains generally have slightly diminished efficiency of enzymatic hydrolysis compared to the corresponding 7-phenylacetamido analog. It was also demonstrated that the less toxic prodrug 3 was activated in an immunologically specific manner by L6-F(ab')-beta-lactamase and 96.5-F(ab')-beta-lactamase conjugates, selective for H2987 and clone 62 cells, respectively.

Cephalosporin derivatives of doxorubicin as prodrugs for activation by monoclonal antibody-beta-lactamase conjugates.

The synthesis of a series of cephalosporin doxorubicin derivatives that differ with respect to the substituent at position 7 of the cephem nucleus is described. These compounds are designed as prodrugs of doxorubicin for activation by monoclonal antibody-beta-lactamase conjugates. The key step in the synthesis of this series of compounds involves the use of the phenylacetamido group as an enzymatically removable protecting group for the 7-amino group on the cephem. In vitro cytotoxicity assays with H2981 lung adenocarcinoma cells revealed that cephalosporin doxorubicin derivatives were all less toxic than the released drug. Prodrugs containing negatively charged groups in the side chain, such as the delta-carboxybutanamido derivative 4 and the alpha-sulfophenylacetyl derivative 5, displayed the least cytotoxic activity and were 46- and 26-fold less toxic than doxorubicin, respectively. The efficiency of activation of all the prodrugs was evaluated in cytotoxicity assays on H2981 cells with the beta-lactamases from Enterobacter cloacae P99, Escherichia coli TEM-1, and Bacillus cereus (type II). In general, the E. cloacae enzyme was found to most rapidly activate the majority of these prodrugs. Phenylacetamido prodrug 2 and delta-carboxybutanamido prodrug 4 were both activated in an immunospecific manner by L6-E. cloacae beta-lactamase, a monoclonal antibody conjugate that binds to receptors on H2981 lung adenocarcinoma cells.

Therapeutic effects of monoclonal antibody-beta-lactamase conjugates in combination with a nitrogen mustard anticancer prodrug in models of human renal cell carcinoma.

A panel of 13 renal cell carcinoma cell lines was evaluated for the expression of antigens recognized by the L6 and L49 monoclonal antibodies. All of the cell lines were strongly positive for the L6 antigen, and 9/13 bound 96.5, which, like the L49 monoclonal antibody, recognizes the p97 melanotransferrin antigen. The L6 and L49 antibodies were chemically conjugated to Enterobacter cloacae beta-lactamase (bL), and their abilities to effect site-selective anticancer prodrug activation on two of the renal cell carcinoma cell lines (SN12P and 1934J) were evaluated in vitro and in vivo. L49-bL was 10-90-fold more potent in vitro than L6-bL for the activation of 7-(4-carboxybutanamido)cephalosporin mustard (CCM), a cephalosporin prodrug of phenylenediamine mustard (PDM). In addition, L49-bL showed higher degrees of specific SN12P and 1934J intratumoral uptake than L6-bL, even though the expression of L6 antigen was 2-fold higher than that of p97. These differences might be due to the high-affinity antigen binding of L49-bL relative to L6-bL. In vivo studies utilizing nude mice with established subcutaneous SN12P and 1934J tumor xenografts demonstrated that L49-bL/CCM combinations led to regressions and cures at well-tolerated doses, while L6-bL/CCM and the nonbinding control conjugate P1.17-bL in combination with CCM were ineffective. Conjugate localization in 1934J tumors was much lower than that observed in SN12P tumors, a finding that might acount for the higher activities of L49-bL/CCM in the latter model. These data show that the p97 antigen on renal cell carcinomas can be exploited for selective prodrug activation, even on tumors that localize very small amounts of the L49-bL conjugate.

Prodrugs of doxorubicin and melphalan and their activation by a monoclonal antibody-penicillin-G amidase conjugate.

The syntheses and cytotoxic activities of substituted N-phenylacetamido derivatives of doxorubicin and melphalan are described. The derivatives were designed as prodrugs which could be activated in a site-specific manner by monoclonal antibody-penicillin-G amidase (mAb-PGA) conjugates. N-(Phenylacetamido)doxorubicin (2) and N-(phenylacetyl)melphalan (6) were found to be 10- and 20-fold less cytotoxic against H2981 lung adenocarcinoma cells than doxorubicin and melphalan, respectively. When incubated with PGA, the cytotoxicity of 2 and 6 increased and became equivalent to that of the corresponding drugs from which they were made. The poor solubility characteristics of 2 in aqueous solutions provided the basis for the development of the more soluble doxorubicin derivatives, N-(4-aminophenylacetyl)doxorubicin (3) and N-(4-phosphonooxy)phenylacetyl)-doxorubicin (4). In vitro cytotoxicity assays indicated that 3 and 4 were at least 1000-fold less toxic than doxorubicin against H2981 cells. PGA and the mAb conjugate L6-PGA were able to effect the activation of 3 and 6 on H2981 cells (L6-antigen positive). Hydrolysis of the phosphate group of 4 was required prior to activation with PGA or L6-PGA. This was achieved using alkaline phosphatase, or by exposing 4 to phosphatases present in cell culture medium. The activation of 3, 4, and 6 on H2981 cells by L6-PGA occurred in an immunologically specific manner, since activation could be blocked by saturating cell surface antigens with L6 prior to treatment with L6-PGA. These results demonstrate that 3, 4, and 6 are prodrugs that can be specifically activated to release clinically approved anticancer agents by a mAb-PGA conjugate.

Effects of a hybrid recombinant human alpha interferon (A/D) on in vitro cytotoxicity and in vivo localization of monoclonal antibody L6-cytosine deaminase conjugate in a colon cancer model.

L6 is an IgG2a murine monoclonal antibody which we have demonstrated binds well to HT29 human colon carcinoma cells by flow cytometry, whole cell ELISA, and mixed hemadsorption. In vitro cytotoxicity studies revealed that the monoclonal antibody L6-cytosine deaminase (L6-CD) immunoconjugate plus the nontoxic prodrug, 5-fluorocytosine (5-FC), is equivalent to 5-fluorouracil (5-FU) in its ability to kill HT29 cells. Human alpha-interferon (A/D) was able to enhance this cytotoxic effect. The I.C.50's revealed that very small amounts of L6-CD are needed for this cytotoxic effect (approximately, 5 pg/ml resulted in 50% viability). The limiting factor was the amount of 5-FC employed with L6-CD (3 microM yielded 50% cell viability). alpha-Interferon (A/D) lowered the requirement of 5-FC to 1 microM to achieve 50% cell lethality. In vivo biodistribution experiments indicated that 1 microgram of L6-CD is nonspecifically taken up by the liver and spleen and cleared rapidly from the blood. Significant localization of L6-CD to HT29 tumors occurred only when 99 micrograms of unlabeled L6-CD was added to 1 microgram of 125I-labeled immunoconjugate injected intravenously. Further augmentation of tumor/blood ratios without reduction in percent injected dose per gram of tumor was possible with the intravenous injection of 100 micrograms of anti-idiotypic monoclonal antibody 13B, 24 hours after L6-CD, which bound unreacted L6-CD and cleared it from the blood. The addition of 100,000 U of alpha-interferon (A/D) given intraperitoneally every day increased the clearance of L6-CD by the liver and spleen, but impaired tumor localization (percent injected dose per gram). These studies demonstrated that in vivo localization of the L6-CD conjugate to HT29 tumors could be optimized by injecting excess L6-CD followed by an equal amount of L6 anti-idiotype mAb 13B, 24 hours after L6-CD.

Reassignment of the structure for the antitumor agent RR-150.

7-Cysteaminomitosane (RR-150) has been reported to be superior to mitomycin C against P388 leukemia and B-16 melanoma in mice and is less leukopenic. Studies reported here indicated the absence of a free thiol group in RR-150 and therefore the structure was incorrectly assigned. Reaction of mitomycin A with either 2-aminoethanethiol or cystamine gave the same disulfide, 7-N,7'-N'-dithiodiethylenedimitomycin C, which is the newly proposed structure for RR-150. Attempts to produce 7-cysteaminomitosane by reduction of the disulfide have not succeeded because of its apparent instability.

Selective activation of anticancer prodrugs by monoclonal antibody-enzyme conjugates.

Several recent reports have demonstrated that anticancer drugs can be generated site-selectively at solid tumors by monoclonal antibody-enzyme conjugates targeted to antigens on tumor cell surfaces. The first step in this drug targeting approach involves the delivery of the enzyme conjugate to a tumor cell population. After the conjugate has localized within the tumor and cleared from non-target tissues, a relatively non-cytotoxic drug precursor (prodrug) is administered. Upon contact with the targeted enzyme, the prodrug is converted into a toxic drug. Several examples are presented to illustrate this targeting strategy. Monoclonal antibody-beta-lactamase conjugates have been developed to activate a panel of anticancer prodrugs that are mechanistically dissimilar. The antitumor activities of the monoclonal antibody-beta-lactamase conjugate/prodrug combinations exceed those obtained by systemic drug administration, and are immunologically specific. In another example involving targeted cytosine deaminase for the generation of 5-fluorouracil, it is shown that as much as 17 times more drug can be delivered within a tumor compared to when 5-fluorouracil is administered alone. The method of using targeted enzymes for prodrug activation can be extended to include prodrugs that release very potent drugs, such as palytoxin, a marine natural product, and to treat cells that have the multidrug resistance phenotype. Some of the requirements for successful therapy with this approach for cancer therapy are discussed.