Cathepsin B-labile dipeptide linkers for lysosomal release of doxorubicin from internalizing immunoconjugates: model studies of enzymatic drug release and antigen-specific in vitro anticancer activity

Designing immunoconjugates for cancer therapy

INTRODUCTION

Antibody-drug conjugates (ADCs), as well as antibody conjugates of protein toxins (immunotoxins) and cytokines (immunocytokines), are showing clinical efficacy, with manageable toxicities, in cancer treatment.

AREAS COVERED

The utility of an ADC is governed by the antibody and the target, as well as by the drug-linker component of the conjugate. The conjugation site, conjugating group, drug/antibody ratios and site-specific conjugation for product homogeneity are all aspects to consider in optimizing the ADC and enhancing its therapeutic window. Immunotoxin and immunocytokine construction by recombinant methods can be modulated to improve efficacy and reduce toxicity. The Dock-and-Lock (DNL) platform technology provides a flexible approach to assemble mono- or bispecific constructs carrying multiple toxin or cytokine molecules for targeted therapy.

EXPERT OPINION

Conjugation chemistry and recombinant technologies have had a significant impact on the therapeutic prospects of immunoconjugates, particularly in hematopoietic diseases. Continued concerted efforts from different scientific disciplines are needed, together with newer treatment paradigms, for greater progress in the more challenging therapy of solid tumors.

Brentuximab vedotin in Hodgkin's lymphoma

INTRODUCTION

Monoclonal antibodies (mAbs) directed against antigens expressed by tumor cells have become an effective therapeutic option in hematologic malignancies. Antibody-drug conjugates (ADC) utilize the targeting properties of mAbs to deliver cytotoxic agents inside the malignant cell. Brentuximab vedotin is a CD30-specific ADC with significant clinical activity in Hodgkin's lymphoma (HL).

AREAS COVERED

CD30 is an ideal target for therapy due to its limited expression on normal tissues and the strong and uniform expression on malignant cells in classical Hodgkin's lymphoma (cHL). Early studies using unconjugated CD30 mAbs have demonstrated lack of objective responses in patients with recurrent cHL. Brentuximab vedotin is an ADC with significant clinical activity in patients with HL. Here, the authors review the biology of brentuximab vedotin, summarize available clinical data and discuss future treatment strategies.

EXPERT OPINION

Brentuximab vedotin is a well-tolerated and highly active drug in patients with relapsed cHL. This novel agent has the potential to be incorporated in the standard management of the disease with significant therapeutic impact.

Development of novel bisphosphonate prodrugs of doxorubicin for targeting bone metastases that are cleaved pH dependently or by cathepsin B: synthesis, cleavage properties, and binding properties to hydroxyapatite as well as bone matrix

Bone metastases are a frequent cause of morbidity in cancer patients. The present palliative therapeutic options are chemotherapy, hormone therapy, and the administration of bisphosphonates. The affinity between bisphosphonates and the apatite structure of bone metastases is strong. Thus, we designed two low-molecular-weight and water-soluble prodrugs which incorporate a bisphosphonate group as a bone targeting ligand, doxorubicin as the anticancer agent, and either an acid-sensitive bond (1) or a cathepsin B cleavable bond (3) for ensuring an effective release of doxorubicin at the site of action. Cleavage studies of both prodrugs showed a fast release of doxorubicin but sufficient stability over several hours in human plasma. Effective binding of prodrug 1 and 3 was demonstrated with hydroxyapatite and with native bone. In orientating toxicity studies in nude mice, the MTD of 1 was 3-fold higher compared to conventional doxorubicin, whereas 3 showed essentially the same MTD as doxorubicin.

Targeting HER2-positive cancer with dolastatin 15 derivatives conjugated to trastuzumab, novel antibody-drug conjugates

PURPOSE

Targeting tubulin binders to cancer cells using antibody-drug conjugates (ADCs) has great potential to become an effective cancer treatment with low normal tissue toxicity. The nature of the linker used to tether the tubulin binder to the antibody and the conjugation sites on the antibody and the small molecule are important factors in the ADC stability and effectiveness.

METHODS

We explored the use of tubulin-targeting dolastatin 15 derivatives (Dol15) tethered covalently to a representative antibody, trastuzumab, via cleavable and non-cleavable linkers at varying antibody reactive sites (i.e., lysine residues, partially reduced hinge region disulfide bonds) and drug coupling sites (i.e., C-terminus, N-terminus), to investigate which constructs were more effective in killing HER2-positive cells in vitro and in vivo.

RESULTS

We found that Dol15 conjugated to trastuzumab via lysine residues at the drug C-terminus using a non-cleavable linker (trastuzumab-amide-C-term-Dol15) produced target-dependent growth inhibition of cells endogenously expressing high HER2 levels (i.e., SK-BR-3, SK-OV-3) in vitro. This ADC was effective at varying doses (i.e., 10 and 20 mg/kg) in the SK-OV-3 human ovarian cancer xenograft.

CONCLUSIONS

Tethering Dol15 via partially reduced disulfide bonds at the drug C-terminus via a non-cleavable linker (trastuzumab-MC-C-term-Dol15) resulted in an equally effective ADC in vitro, showing that site of antibody conjugation did not influence ADC activity. However, tethering Dol15 at the drug N-terminus using non-cleavable and cleavable linkers (trastuzumab-MC-N-term-Dol15 and trastuzumab-MC-VC-PABC-N-term-Dol15, respectively) resulted in ineffective ADCs. Thus, Dol15 tethered at the C-terminus may be a useful tubulin-targeting agent for conjugation at various antibody reactive sites.

Synthesis and biological characterization of protease-activated prodrugs of doxazolidine

Doxazolidine (doxaz) is a new anthracycline anticancer agent. While structurally similar to doxorubicin (dox), doxaz acts via a distinct mechanism to selectively enhance anticancer activity over cardiotoxicity, the most significant clinical impediment to successful anthracycline treatment. Here, we describe the synthesis and characterization of a prodrug platform designed for doxaz release mediated by secreted proteolytic activity, a common association with invasiveness and poor prognosis in cancer patients. GaFK-Doxaz is hydrolyzable by the proteases plasmin and cathepsin B, both strongly linked with cancer progression, as well as trypsin. We demonstrate that activation of GaFK-Doxaz releases highly potent doxaz that powerfully inhibits the growth of a wide variety of cancer cells (average IC(50) of 8 nM). GaFK-Doxaz is stable in human plasma and is poorly membrane permeable, thereby limiting activation to locally secreted proteolytic activity and reducing the likelihood of severe side effects.

Photocaged permeability: a new strategy for controlled drug release

Light is used to release a drug from a cell impermeable small molecule, uncloaking its cytotoxic effect on cancer cells.

Cathepsin B-sensitive polymers for compartment-specific degradation and nucleic acid release

Degradable cationic polymers are desirable for in vivo nucleic acid delivery because they offer significantly decreased toxicity over non-degradable counterparts. Peptide linkers provide chemical stability and high specificity for particular endopeptidases but have not been extensively studied for nucleic acid delivery applications. In this work, enzymatically degradable peptide-HPMA copolymers were synthesized by RAFT polymerization of HPMA with methacrylamido-terminated peptide macromonomers, resulting in polymers with low polydispersity and near quantitative incorporation of peptides. Three peptide-HPMA copolymers were evaluated: (i) pHCathK(10), containing peptides composed of the linker phe-lys-phe-leu (FKFL), a substrate of the endosomal/lysosomal endopeptidase cathepsin B, connected to oligo-(L)-lysine for nucleic acid binding, (ii) pHCath(D)K(10), containing the FKFL linker with oligo-(D)-lysine, and (iii) pH(D)Cath(D)K(10), containing all (D) amino acids. Cathepsin B degraded copolymers pHCathK(10) and pHCath(D)K(10) within 1 h while no degradation of pH(D)Cath(D)K(10) was observed. Polyplexes formed with pHCathK(10) copolymers show DNA release by 4 h of treatment with cathepsin B; comparatively, polyplexes formed with pHCath(D)K(10) and pH(D)Cath(D)K(10) show no DNA release within 8 h. Transfection efficiency in HeLa and NIH/3T3 cells were comparable between the copolymers but pHCathK(10) was less toxic. This work demonstrates the successful application of peptide linkers for degradable cationic polymers and DNA release.

Cathepsin B cleavable novel prodrug Ac-Phe-Lys-PABC-ADM enhances efficacy at reduced toxicity in treating gastric cancer peritoneal carcinomatosis: an experimental study

BACKGROUND

Doxorubicin (Adriamycin) is effective in gastric cancer treatment, but with severe dose-dependent toxicities. A novel prodrug of doxorubicin (Ac-Phe-Lys-PABC-ADM) is designed to deliver free doxorubicin relying on cathepsin B and reduce side effects. The authors examined the antitumor effect and toxicities of Ac-Phe-Lys-PABC-ADM against gastric cancer peritoneal carcinomatosis.

METHODS

SGC-7901 gastric cancer cell line was used for the study. The in vitro study investigated the effects of doxorubicin and Ac-Phe-Lys-PABC-ADM on cell growth dynamics and cell cycle. The in vivo study investigated the efficacy and toxicity of Ac-Phe-Lys-PABC-ADM on a nude mice model of peritoneal carcinomatosis, with doxorubicin as positive control.

RESULTS

In the in vitro study, Ac-Phe-Lys-PABC-ADM had a lower dose-dependent inhibitory effect on SGC-7901 cells. In the in vivo study of control, doxorubicin, and Ac-Phe-Lys-PABC-ADM groups, the median experimental peritoneal carcinomatosis indexes were 6, 1.5, and 1, respectively (P = .004); the body weights were 24.32 ± 1.40 g, 18.40 ± 2.97 g, and 23.61 ± 0.80 g, respectively (P = .000). Biochemical studies showed that Ac-Phe-Lys-PABC-ADM had significantly lower toxicities on the bone marrow, liver, kidney, and particularly heart. Histopathological studies of the control, doxorubicin, and Ac-Phe-Lys-PABC-ADM groups found significant myocardium toxicities in 3, 7, and 4 animals, respectively.

CONCLUSIONS

Ac-Phe-Lys-PABC-ADM could be an effective molecular targeting drug to treat gastric cancer peritoneal carcinomatosis with enhanced efficacy and reduced toxicity.

Immunotoxins and anticancer drug conjugate assemblies: the role of the linkage between components

Immunotoxins and antibody-drug conjugates are protein-based drugs combining a target-specific binding domain with a cytotoxic domain. Such compounds are potentially therapeutic against diseases including cancer, and several clinical trials have shown encouraging results. Although the targeted elimination of malignant cells is an elegant concept, there are numerous practical challenges that limit conjugates’ therapeutic use, including inefficient cellular uptake, low cytotoxicity, and off-target effects. During the preparation of immunoconjugates by chemical synthesis, the choice of the hinge component joining the two building blocks is of paramount importance: the conjugate must remain stable in vivo but must afford efficient release of the toxic moiety when the target is reached. Vast efforts have been made, and the present article reviews strategies employed in developing immunoconjugates, focusing on the evolution of chemical linkers.

Humanized anti-Trop-2 IgG-SN-38 conjugate for effective treatment of diverse epithelial cancers: preclinical studies in human cancer xenograft models and monkeys

PURPOSE

Evaluate the efficacy of an SN-38-anti-Trop-2 antibody-drug conjugate (ADC) against several human solid tumor types, and to assess its tolerability in mice and monkeys, the latter with tissue cross-reactivity to hRS7 similar to humans.

EXPERIMENTAL DESIGN

Two SN-38 derivatives, CL2-SN-38 and CL2A-SN-38, were conjugated to the anti-Trop-2-humanized antibody, hRS7. The immunoconjugates were characterized in vitro for stability, binding, and cytotoxicity. Efficacy was tested in five different human solid tumor-xenograft models that expressed Trop-2 antigen. Toxicity was assessed in mice and in Cynomolgus monkeys.

RESULTS

The hRS7 conjugates of the two SN-38 derivatives were equivalent in drug substitution (∼ 6), cell binding (K(d) ∼ 1.2 nmol/L), cytotoxicity (IC(50) ∼ 2.2 nmol/L), and serum stability in vitro (t/(½) ∼ 20 hours). Exposure of cells to the ADC demonstrated signaling pathways leading to PARP cleavage, but differences versus free SN-38 in p53 and p21 upregulation were noted. Significant antitumor effects were produced by hRS7-SN-38 at nontoxic doses in mice bearing Calu-3 (P ≤ 0.05), Capan-1 (P < 0.018), BxPC-3 (P < 0.005), and COLO 205 tumors (P < 0.033) when compared to nontargeting control ADCs. Mice tolerated a dose of 2 × 12 mg/kg (SN-38 equivalents) with only short-lived elevations in ALT and AST liver enzyme levels. Cynomolgus monkeys infused with 2 × 0.96 mg/kg exhibited only transient decreases in blood counts, although, importantly, the values did not fall below normal ranges.

CONCLUSIONS

The anti-Trop-2 hRS7-CL2A-SN-38 ADC provides significant and specific antitumor effects against a range of human solid tumor types. It is well tolerated in monkeys, with tissue Trop-2 expression similar to humans, at clinically relevant doses, and warrants clinical investigation.

Design, Synthesis, and Antitumor Activity of 4-Halocolchicines and Their Pro-drugs Activated by Cathepsin B

Novel colchicine derivatives possessing various substituents at the C4 position were prepared. Among them, 4-halo derivatives 3-6 were found to exhibit higher activity against cancer cell lines (A549, HT29, HCT116) as well as on mice transplanted with the HCT116 human colorectal carcinoma cell line than colchicine (1). Further, utilizing the 4-substituted colchicines, we prepared pro-drugs having a dipeptide side chain and demonstrated that these pro-drugs were activated by cathepsin B, an enzyme overexpressed in tumor cells, and exhibited selective toxicity to the tumor cells.

The hexapeptide PGVTAV suppresses neurotoxicity of human α-synuclein aggregates

In Parkinson's disease patients, α-synuclein is the major component of the intracellular protein aggregates found in dopaminergic neurons. Previously, short synthetic α-synuclein-derived peptides have been shown to not only prevent α-synuclein fibrillation but also dissolve preformed α-synuclein aggregates in vitro. The hexapeptide PGVTAV was the shortest peptide that retained the ability to block α-synuclein fibrillation. For preventative or therapeutic effectiveness, a treatment must suppress the neurotoxicity of α-synuclein aggregates and remain stable in plasma. The present study shows that specific peptides can protect neuronal cells from α-synuclein aggregation-induced cell death. The β-sheet-breaking hexapeptide PGVTAV remained intact in human plasma for longer than one day, suggesting that it may be a candidate for the development of therapeutics to treat Parkinson's disease.

Cancer-targeting Antibody–Drug Conjugates: Site-specific Conjugation of Doxorubicin to Anti-EGFR 528 Fab' through a Polyethylene Glycol Linker

Antibody–drug conjugates have been prepared to examine the effect that attaching small-molecule drugs to an antibody fragment has on antibody activity. The anticancer drug doxorubicin was covalently attached through a polyethylene glycol linker to a cancer-targeting, anti-epidermal growth factor receptor antibody fragment (Fab′). The reactivity of maleimide was compared with a substituted maleimide derivative (citraconimide) in conjugation reactions with cysteine residues on a Fab′. Introduction of polyethylene glycol increased aqueous solubility of the cytotoxic drug, which led to an improvement in overall yield of the conjugation reaction with the antibody fragment. Antibody–drug conjugates prepared retained activity of the parent antibody, as determined by antigen binding experiments measured by surface plasmon resonance.

In vitro degradation and antitumor activity of oxime bond-linked daunorubicin-GnRH-III bioconjugates and DNA-binding properties of daunorubicin-amino acid metabolites

Bioconjugates with receptor-mediated tumor-targeting functions and carrying cytotoxic agents should enable the specific delivery of chemotherapeutics to malignant tissues, thus increasing their local efficacy while limiting the peripheral toxicity. In the present study, gonadotropin-releasing hormone III (GnRH-III; Glp-His-Trp-Ser-His-Asp-Trp-Lys-Pro-Gly-NH(2)) was employed as a targeting moiety to which daunorubicin was attached via oxime bond, either directly or by insertion of a GFLG or YRRL tetrapeptide spacer. The in vitro antitumor activity of the bioconjugates was determined on MCF-7 human breast and HT-29 human colon cancer cells by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Their degradation/stability (1) in human serum, (2) in the presence of cathepsin B and (3) in rat liver lysosomal homogenate was analyzed by liquid chromatography in combination with mass spectrometry. The results show that (1) all synthesized bioconjugates have in vitro antitumor effect, (2) they are stable in human serum at least for 24 h, except for the compound containing an YRRL spacer and (3) they are hydrolyzed by cathepsin B and in the lysosomal homogenate. To investigate the relationship between the in vitro antitumor activity and the structure of the bioconjugates, the smallest metabolites produced in the lysosomal homogenate were synthesized and their binding to DNA was assessed by fluorescence spectroscopy. Our data indicate that the incorporation of a peptide spacer in the structure of oxime bond-linked daunorubicin-GnRH-III bioconjugates is not required for their antitumor activity. Moreover, the antitumor activity is influenced by the structure of the metabolites (daunorubicin-amino acid derivatives) and their DNA-binding properties.

Conjugates of a novel 7-substituted camptothecin with RGD-peptides as α(v)β₃ integrin ligands: An approach to tumor-targeted therapy

Eight conjugates of a novel camptothecin derivative (Namitecan, NMT) with RGD peptides have been synthesized and biologically evaluated. This study focused on factors that optimize the drug linkage to the transport vector. The different linkages investigated consist of heterofunctional glycol fragments and a lysosomally cleavable peptide. The linkage length and conformation were systematically modified with the purpose to understand their effect on receptor affinity, systemic stability, cytotoxicity, and solubility of the corresponding conjugates. Among the new conjugates prepared, C6 and C7 showed high receptor affinity and tumor cell adhesion, acceptable stability in murine blood, and high cytotoxic activity (IC₅₀ = 8 nM). The rationale, synthetic strategy, and preliminary biological results will be presented.

Anti-CD30 Antibodies for Hodgkin lymphoma

Treatment of refractory or relapsed classical Hodgkin lymphoma (HL) remains challenging, but targeted immunotherapy has recently emerged as a potential treatment option for these patients. Although first-generation monoclonal anti-CD30 antibodies proved disappointing, current efforts to modify anti-CD30 antibodies to improve binding of effector cells and enhance activity appears more promising, as does the development of novel antibody-drug conjugates (ADCs). ADCs offer the potential to deliver potent therapies with minimal toxicity. One highly active ADC, brentuximab vedotin (SGN-35), combines an anti-CD30 monoclonal antibody and the antitubulin agent monomethyl auristatin E. Initial phase 1 studies of brentuximab vedotin showed a 52% overall response rate in relapsed HL, with minimal toxicity. This article highlights the development of anti-CD30 antibodies and ADCs for relapsed or refractory classical HL.

Antibody-drug conjugates: linking cytotoxic payloads to monoclonal antibodies

Antibody-drug conjugates (ADCs) combine the specificity of monoclonal antibodies (mAbs) with the potency of cytotoxic molecules, thereby taking advantage of the best characteristics of both components. Along with the development of the mAbs and cytotoxins, the design of chemical linkers to covalently bind these building blocks is making rapid progress but remains challenging. Recent advances have resulted in linkers having increased stability in the bloodstream while allowing efficient payload release within the tumor cell.

Guanylyl cyclase C as a biomarker for targeted imaging and therapy of metastatic colorectal cancer

The guanylyl cyclase C (GCC) receptor posseses several well-established properties ideal for use as a biomarker in gastrointestinal malignancies. The GCC receptor is constitutively expressed in the apical membranes of the intestine and its expression is universally preserved in primary colorectal tumors and their metastases. Moreover, receptor binding is retained by GCC’s cognate ligand, the bacterial enterotoxin ST, even after conjugation to functional moieties. Selective tumor, but not gastrointestinal, uptake of ST in mice bearing GCC-expressing colon cancer xenografts demonstrates the potential of exploiting ST–GCC interaction for diagnostic imaging and targeted therapy of metastatic colorectal cancer. We expect this specific targeting provided by ST–GCC interaction to improve diagnosis, staging and management of colorectal cancer metastases, and ultimately prolong patient survival in this disease.

Antibody-cytotoxic agent conjugates: preparation and characterization

Conjugates of antibodies with cytotoxic agents offer a targeted therapeutic strategy against cancer cells expressing target antigens. Several antibodies against various cancer cell-surface antigens have been conjugated with different cytotoxic agents that inhibit essential cellular targets such as microtubules or DNA. Antibody-cytotoxic agent conjugates (ACCs) against several types of cancer are currently in advanced stages of clinical trials and one, gemtuzumab ozogamicin (Mylotarg), is approved for the treatment of acute myeloid leukemia. The linker group connecting the antibody to the cytotoxic agent is an important feature of the ACC, modulating the release of the active cytotoxic agent in the targeted cell. Several linker strategies employed for ACCs in current clinical trials include cleavable linkers with disulfide, hydrazone, lysosomal protease-substrate groups, and non-cleavable linkers. This chapter describes the methods of preparation of conjugates of antibodies with small-molecule cytotoxic agents (maytansinoids, calicheamicin, and auristatins) bearing different linkers. Methods to evaluate the in vitro cytotoxicity and in vivo anti-tumor efficacy of ACC are described in brief. Analytical methods are described to evaluate the mechanism of cellular processing of the ACCs with different linkers and the generation of the active metabolites.

Antibody conjugates of 7-ethyl-10-hydroxycamptothecin (SN-38) for targeted cancer chemotherapy

CPT-11 is a clinically used cancer drug, and it is a prodrug of the potent topoisomerase I inhibitor, SN-38 (7-ethyl-10-hydroxycamptothecin). To bypass the need for the in vivo conversion of CPT-11 and increase the therapeutic index, bifunctional derivatives of SN-38 were prepared for use in antibody-based targeted therapy of cancer. The general synthetic scheme incorporated an acetylene-azide click cycloaddition step in the design, a short polyethylene glycol spacer for aqueous solubility, and a maleimide group for conjugation. Conjugates of a humanized anti-CEACAM5 monoclonal antibody, hMN-14, prepared using these SN-38 derivatives were evaluated in vitro for stability in buffer and human serum and for antigen-binding and cytotoxicity in a human colon adenocarcinoma cell line. Conjugates of hMN-14 and SN-38 derivatives 16 and 17 were found promising for further development.

Use of antibodies and immunoconjugates for the therapy of more accessible cancers

There are currently 6 unconjugated antibodies and 3 immunoconjugates approved for use in the United States in a variety of cancers, with a considerable number of new agents in clinical testing and preclinical development. Unconjugated antibodies alone can be effective, but more often, antibodies need to be combined with chemotherapy, which enhances the efficacy of the standard treatment. Immunoconjugates tend to be more effective than their unconjugated counterparts, but their increased toxicity often restricts when and how they are used. In order to improve efficacy, a number of immunoconjugates are being examined in settings where the disease is more easily accessible, such as leukemias, or within compartments that allow easier and more direct access to the tumor, such as in the peritoneal cavity or brain, or both locally and systemically, in adjuvant situations, where the disease burden has been reduced by some other means, and with the main goal of these treatments being to kill residual disease.

Peptide-targeted polyglutamic acid doxorubicin conjugates for the treatment of alpha(v)beta(6)-positive cancers

Most chemotherapeutics exert their effects on tumor cells as well as their healthy counterparts, resulting in dose limiting side effects. Cell-specific delivery of therapeutics can increase the therapeutic window for treatment by maintaining the therapeutic efficacy while decreasing the untoward side effects. We have previously identified a peptide, named H2009.1, which binds to the integrin alpha(v)beta(6). Here, we report the synthesis of a peptide targeted polyglutamic acid polymer in which the high affinity alpha(v)beta(6)-specific tetrameric H2009.1 peptide is incorporated via a thioether at the N-terminus of a 15 amino acid polymer of glutamic acid. Doxorubicin is incorporated into the polymer via an acid-labile hydrazone bond. Payloads of four doxorubicin molecules per targeting agent are achieved. The drug is released at pH 4.0 and 5.6 but the conjugate is stable at pH 7.0. The conjugate is selectively internalized into alpha(v)beta(6) positive cells as witnessed by flow cytometric analysis and fluorescent microscopy. Cellular uptake is mediated by the H2009.1 peptide, as no internalization of the doxorubicin-PG polymer is observed when it is conjugated to a scrambled sequence control peptide. Importantly, the conjugate is more cytotoxic toward a targeted cell than a cell line that does not express the integrin.

CD133/prominin-1 is a potential therapeutic target for antibody-drug conjugates in hepatocellular and gastric cancers

CD133/prominin-1 is a pentaspan transmembrane glycoprotein overexpressed in various solid tumours including colorectal and glioblastomas. CD133 was found here to be highly expressed in ⩾50% of pancreatic, gastric and intrahepatic cholangiocarcinomas. Quantitative flow cytometric analysis showed that a panel of established hepatocellular, pancreatic and gastric cancer cell lines expressed CD133 at levels higher than normal epithelial cells or bone marrow progenitor cells. A murine anti-human CD133 antibody (AC133) conjugated to a potent cytotoxic drug, monomethyl auristatin F (MMAF), effectively inhibited the growth of Hep3B hepatocellular and KATO III gastric cancer cells in vitro with IC₅₀ values of 2–7 ng ml⁻¹. MMAF induced apoptosis in the cancer cells as measured by caspase activation. The anti-CD133-drug conjugate (AC133-vcMMAF) was shown to internalise and colocalised with the lysosomal marker CD107a in the sensitive cell lines. In contrast, in the resistant cell line Su.86.86, the conjugate internalised and colocalised with the caveolae marker, Cav-1. Addition of ammonium chloride, an inhibitor of lysosomal trafficking and processing, suppressed the cytotoxic effect of AC133-vcMMAF in both Hep3B and KATO III. Anti-CD133-drug conjugate treatment resulted in significant delay of Hep3B tumour growth in SCID mice. Anti-CD133 antibody-drug conjugates warrant further evaluation as a therapeutic strategy to eradicate CD133+ tumours.

A cell-penetrating peptidic GRP78 ligand for tumor cell-specific prodrug therapy

Tumor targeting peptides are promising vehicles for site-directed cancer therapy. Pep42, a cyclic 13-mer oligopeptide that specifically binds to glucose-regulated protein 78 (GRP78) and internalized into cancer cells, represents an excellent vehicle for tumor cell-specific chemotherapy. Here, we report the synthesis and evaluation of Pep42-prodrug conjugates that contain a cathepsin B-cleavable linker, resulting in the traceless release of drug inside the cancer cells.

Albumin-Binding Prodrugs of Camptothecin and Doxorubicin with an Ala-Leu-Ala-Leu-Linker That Are Cleaved by Cathepsin B: Synthesis and Antitumor Efficacy

We have recently validated a macromolecular prodrug strategy for improved cancer chemotherapy based on two features: (a) rapid and selective binding of thiol-reactive prodrugs to the cysteine-34 position of endogenous albumin and (b) acid-sensitive promoted or enzymatic release of the drug at the tumor site [Kratz, F., Warnecke, A., Scheuemann, K., Stockmar, C., Schwab, J., Lazar, P., Druckes, P., Esser, N., Drevs, J., Rognan, D., Bissantz, C., Hinderling, C., Folkers, G., Fichtner, I., and Unger, C. (2002) J. Med. Chem. 45, 5523-33]. In the present work, we developed water-soluble camptothecin (CPT) and doxorubicin (DOXO) prodrugs that incorporate the peptide linker Ala-Leu-Ala-Leu that serves as a substrate for the tumor-associated protease, cathepsin B, which is overexpressed in several solid tumors. Consequently, two albumin-binding prodrugs were synthesized [EMC-Arg-Arg-Ala-Leu-Ala-Leu-Ala-CPT (1) and EMC-Arg-Arg-Ala-Leu-Ala-Leu-DOXO (2) (EMC = 6-maleimidocaproic acid)]. Both prodrugs exhibited excellent water-solubility and bound rapidly and selectively to the cysteine-34 position of endogenous albumin. Further in vitro studies showed that the albumin-bound form of the prodrugs was cleaved specifically by cathepsin B as well as in human tumor homogenates. Major cleavage products were CPT-peptide derivatives and CPT for the CPT prodrug and H-Leu-Ala-Leu-DOXO, H-Leu-DOXO, and DOXO for the doxorubicin prodrug. In vivo, 1 was superior to free camptothecin in an HT-29 human colon xenograft model; the antitumor efficacy of prodrug 2 was comparable to that of free doxorubicin in the M-3366 mamma carcinoma xenograft model at equimolar doses.

Minor groove binder antibody conjugates employing a water soluble beta-glucuronide linker

The minor groove binder beta-glucuronide drug-linker 3 was constructed from amino CBI 1 and determined to be a substrate for Escherichia coli beta-glucuronidase (EC 3.2.1.31), resulting in facile drug release. Compound 3 was conjugated to mAbs cAC10 (anti-CD30) and h1F6 (anti-CD70) to give antibody-drug conjugates (ADCs) with potencies comparable to that of free drug 1. The ADCs were largely monomeric at intermediate loading levels (4-5drug/mAb), in contrast to highly aggregated p-aminobenzylcarbamate dipeptide-based ADCs of 1 previously reported. Significant levels of immunologic specificity were observed with cAC10-3 by comparing antigen positive versus negative cell lines and binding versus non-binding control ADCs. The water soluble beta-glucuronide linker is stable in plasma and effectively delivers drugs to target cells leading to potent cytotoxic activities.

Evaluation of RGD-Targeted Albumin Carriers for Specific Delivery of Auristatin E to Tumor Blood Vessels

Induction of apoptosis in endothelial cells is considered an attractive strategy to therapeutically interfere with a solid tumor's blood supply. In the present paper, we constructed cytotoxic conjugates that specifically target angiogenic endothelial cells, thus preventing typical side effects of apoptosis-inducing drugs. For this purpose, we conjugated the potent antimitotic agent monomethyl-auristatin-E (MMAE) via a lysosomal cleavable linker to human serum albumin (HSA) and further equipped this drug-albumin conjugate with cyclic c(RGDfK) peptides for multivalent interaction with alphavbeta3-integrin. The RGD-peptides were conjugated via either an extended poly(ethylene glycol) linker or a short alkyl linker. The resulting drug-targeting conjugates RGDPEG-MMAE-HSA and RGD-MMAE-HSA demonstrated high binding affinity and specificity for alphavbeta3-integrin expressing human umbilical vein endothelial cells (HUVEC). Both types of conjugates were internalized by endothelial cells and killed the target cells at low nM concentrations. Furthermore, we observed RGD-dependent binding of the conjugates to C26 carcinoma. Upon i.v. administration to C26-tumor bearing mice, both drug-targeting conjugates displayed excellent tumor homing properties. Our results demonstrate that RGD-modified albumins are suitable carriers for cell selective intracellular delivery of cytotoxic compounds, and further studies will be conducted to assess the antivascular and tumor inhibitory potential of RGDPEG-MMAE-HSA and RGD-MMAE-HSA.

Development and Properties of β-Glucuronide Linkers for Monoclonal Antibody−Drug Conjugates

A beta-glucuronide-based linker for attaching cytotoxic agents to monoclonal antibodies (mAbs) was designed and evaluated. We employed the cytotoxic auristatin derivatives MMAE (1a) and MMAF (1b) and doxorubicin propyloxazoline (DPO, 2) to give the beta-glucuronide drug-linkers 9a, 9b, and 17, respectively. Cysteine-quenched derivatives of 9b and 17 were determined to be substrates for E. coli beta-glucuronidase, resulting in facile drug release. The beta-glucuronide MMAF drug-linker 9b was highly stable in rat plasma with an extrapolated half-life of 81 days. Each drug-linker when conjugated to mAbs c1F6 (anti-CD70) and cAC10 (anti-CD30) gave monomeric antibody-drug conjugates (ADCs) with as many as eight drugs per mAb and had high levels of immunologically specific cytotoxic activity on cancer cell lines. cAC10-9a displayed pronounced antitumor activity in a subcutaneous Karpas 299 lymphoma tumor model. A single dose treatment led to cures in all animals at the 0.5 mg/kg dose level and above, and the conjugate was well tolerated at 100 mg/kg. In mice with subcutaneous renal cell carcinoma xenografts, the MMAF conjugate c1F6-9b was tolerated at 25 mg/kg and efficacious at 0.75 mg/kg. These results demonstrate that the beta-glucuronide linker system is an effective strategy for targeting cytotoxic agents providing ADCs with high degrees of efficacy at well-tolerated doses.

Lysosomal trafficking and cysteine protease metabolism confer target-specific cytotoxicity by peptide-linked anti-CD30-auristatin conjugates

The chimeric anti-CD30 monoclonal antibody cAC10, linked to the antimitotic agents monomethyl auristatin E (MMAE) or F (MMAF), produces potent and highly CD30-selective anti-tumor activity in vitro and in vivo. These drugs are appended via a valine-citrulline (vc) dipeptide linkage designed for high stability in serum and conditional cleavage and putative release of fully active drugs by lysosomal cathepsins. To characterize the biochemical processes leading to effective drug delivery, we examined the intracellular trafficking, internalization, and metabolism of the parent antibody and two antibody-drug conjugates, cAC10vc-MMAE and cAC10vc-MMAF, following CD30 surface antigen interaction with target cells. Both cAC10 and its conjugates bound to target cells and internalized in a similar manner. Subcellular fractionation and immunofluorescence studies demonstrated that the antibody and antibody-drug conjugates entering target cells migrated to the lysosomes. Trafficking of both species was blocked by inhibitors of clathrin-mediated endocytosis, suggesting that drug conjugation does not alter the fate of antibody-antigen complexes. Incubation of cAC10vc-MMAE or cAC10vc-MMAF with purified cathepsin B or with enriched lysosomal fractions prepared by subcellular fractionation resulted in the release of active, free drug. Cysteine protease inhibitors, but not aspartic or serine protease inhibitors, blocked antibody-drug conjugate metabolism and the ensuing cytotoxicity of target cells and yielded enhanced intracellular levels of the intact conjugates. These findings suggest that in addition to trafficking to the lysosomes, cathepsin B and perhaps other lysosomal cysteine proteases are requisite for drug release and provide a mechanistic basis for developing antibody-drug conjugates cleavable by intracellular proteases for the targeted delivery of anti-cancer therapeutics.

Dipeptide-based highly potent doxorubicin antibody conjugates

Highly potent and novel derivatives of doxorubicin were linked to monoclonal antibodies (mAbs) for site-specific drug delivery. Drug linker 5 consisted of a dipeptide linker attached directly to the daunosamine nitrogen of the n-butyldiacetate doxorubicin derivative 2a. Upon hydrolysis of the peptide linker and acetate groups, the free daunosamine nitrogen is able to form the highly potent 2-pyrrolinodoxorubicin (3a). The second approach involved the use of an oxazolidine carbamate (13) to mask an activating aldehyde group until proteolytic hydrolysis releases 3a. Both drug linkers were shown to be substrates for the lysosomal enzyme cathepsin B. Each molecule was conjugated to the mAbs c1F6 (anti-CD70) and cAC10 (anti-CD30) to give potent drug conjugates against renal cell carcinoma and anaplastic large cell lymphoma cell lines, respectively. The activities were immunologically selective, since antigen negative cell lines were much less sensitive to treatment with the drug conjugates. The approaches described here for attaching highly potent doxorubicin derivatives to mAbs are novel and allow for control of drug stability while covalently bound to the delivery agent.

Enhanced Activity of Monomethylauristatin F through Monoclonal Antibody Delivery: Effects of Linker Technology on Efficacy and Toxicity

We have previously shown that antibody-drug conjugates (ADCs) consisting of cAC10 (anti-CD30) linked to the antimitotic agent monomethylauristatin E (MMAE) lead to potent in vitro and in vivo activities against antigen positive tumor models. MMAF is a new antimitotic auristatin derivative with a charged C-terminal phenylalanine residue that attenuates its cytotoxic activity compared to its uncharged counterpart, MMAE, most likely due to impaired intracellular access. In vitro cytotoxicity studies indicated that mAb-maleimidocaproyl-valine-citrulline-p-aminobenzyloxycarbonyl-MMAF (mAb-L1-MMAF) conjugates were >2200-fold more potent than free MMAF on a large panel of CD30 positive hematologic cell lines. As with cAC10-L1-MMAE, the corresponding MMAF ADC induced cures and regressions of established xenograft tumors at well tolerated doses. To further optimize the ADC, several new linkers were generated in which various components within the L1 linker were either altered or deleted. One of the most promising linkers contained a noncleavable maleimidocaproyl (L4) spacer between the drug and the mAb. cAC10-L4-MMAF was approximately as potent in vitro as cAC10-L1-MMAF against a large panel of cell lines and was equally potent in vivo. Importantly, cAC10-L4-MMAF was tolerated at >3 times the MTD of cAC10-L1-MMAF. LCMS studies indicated that drug released from cAC10-L4-MMAF was the cysteine-L4-MMAF adduct, which likely arises from mAb degradation within the lysosomes of target cells. This new linker technology appears to be ideally suited for drugs that are both relatively cell-impermeable and tolerant of substitution with amino acids. Thus, alterations of the linker have pronounced impacts on toxicity and lead to new ADCs with greatly improved therapeutic indices.

Recent advances in tumor-targeting anticancer drug conjugates

Traditional cancer chemotherapy relies on the premise that rapidly proliferating cancer cells are more likely to be a killed by cytotoxic agent. In reality, however, cytotoxic agents have very little or no specificity, which leads to systemic toxicity, causing severe undesirable side effects. Therefore, various drug delivery protocols and systems have been explored in the last three decades. Tumor cells overexpress many receptors and biomarkers, which can be used as targets to deliver cytotoxic agents into tumors. In general, a tumor-targeting drug delivery system consists of a tumor recognition moiety and a cytotoxic warhead connected directly or through a suitable linker to form a conjugate. The conjugate, which can be regarded as 'prodrug', should be systemically non-toxic. This means that the linker must be stable in circulation. Upon internalization into the cancer cell the conjugate should be readily cleaved to regenerate the active cytotoxic agent. Tumor-targeting conjugates bearing cytotoxic agents can be classified into several groups based on the type of cancer recognition moieties. This review describes recent advances in tumor-targeting drug conjugates including monoclonal antibodies, polyunsaturated fatty acids, folic acid, hyaluronic acid, and oligopeptides as tumor-targeting moieties.