Antibody-drug conjugates utilize the antibody as a delivery vehicle for highly potent cytotoxic molecules with specificity for tumor-associated antigens for cancer therapy. Critical parameters that govern successful antibody-drug conjugate development for clinical use include the selection of the tumor target antigen, the antibody against the target, the cytotoxic molecule, the linker bridging the cytotoxic molecule and the antibody, and the conjugation chemistry used for the attachment of the cytotoxic molecule to the antibody. Advancements in these core antibody-drug conjugate technology are reflected by recent approval of Adectris? (anti-CD30-drug conjugate) and Kadcyla? (anti-HER2 drug conjugate). The potential approval of an anti-CD22 conjugate and promising new clinical data for anti-CD19 and anti-CD33 conjugates are additional advancements. Enrichment of antibody-drug conjugates with newly developed potent cytotoxic molecules and linkers are also in the pipeline for various tumor targets. However, the complexity of antibody-drug conjugate components, conjugation methods, and off-target toxicities still pose challenges for the strategic design of antibody-drug conjugates to achieve their fullest therapeutic potential. This review will discuss the emergence of clinical antibody-drug conjugates, current trends in optimization strategies, and recent study results for antibody-drug conjugates that have incorporated the latest optimization strategies. Future challenges and perspectives toward making antibody-drug conjugates more amendable for broader disease indications are also discussed.
The most commonly used treatment for cancer is combination therapy, such as surgery with chemotherapy, radiotherapy, or targeted immunotherapy. Since cancer is often present as a disseminated disease, it is imperative to target not only the primary tumor cells but also the distant metastases, without harming non-tumor cells. Therefore, targeted therapy for the tumor-specific antigens has become an invaluable tool in cancer therapy. In particular, antibody-based immunotherapies using monoclonal antibodies (mAbs) and antibody fragments have been the focus of the development of strategic anticancer drugs for many years. The mAbs and their derivatives, such as radionuclides, toxins, or cytotoxic molecule-labeled mAbs have become established as a new drug class for use in targeted cancer therapy. The significance of therapeutic antibodies is, in part, reflected by recent nomenclature regulations for antibody-based drugs as implemented by the International Nonproprietary Names and the United States Adopted Names.
Clinical validation of therapeutic antibodies in combination with chemotherapy or another therapeutic antibody with a different mode of action is now becoming one of the standard therapeutic goals for exploratory drug delivery protocols in clinical oncology. This suggests the insufficient antitumor efficacy of the naked antibody when it is used alone. In an attempt to further improve clinical benefits for patients, the number of antibody-drug conjugates (ADCs), where the tumor antigen-specific antibody is conjugated to the potent cytotoxic molecule, has been rapidly growing, and may provide further promising treatments for cancer. Currently, approximately 45 ADCs are in clinical trials against ∼35 targets, and ∼70% of the therapeutic modalities in Phase I clinical trials are ADCs.
The principle of the ADC is quite simple; however, satisfactory efficacy of therapeutic ADCs has been more difficult to achieve than previously anticipated, as exemplified by the number of ADCs that have been terminated during clinical trial phases. Therefore, optimization of ADCs, and identification of novel therapeutic combinations are needed to further improve the efficacy of immunotherapy. The progress in cancer therapy and emergence of ADCs as drug delivery vehicles for targeted immunotherapy are described in this review. In particular, the critical features of ADCs that contribute to the successful development and clinical implementation of their use, as well as the challenges and latest optimization strategies for therapeutic ADCs are reviewed. Results from current clinical and preclinical studies are also presented.
Numerous cytotoxic molecules have been approved for use as chemotherapeutic agents. Most chemotherapeutic drugs target both proliferating cancer and normal cells, and are used near their maximum tolerated dose (MTD) to achieve therapeutic effects. As a result, the standard therapeutic modality for chemotherapy is often a combination therapy: chemotherapy regimens of CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisolone) for non-Hodgkin’s lymphoma (NHL) and CMF (cyclophosphamide, methotrexate, and 5-fluorouracil) for breast cancer are examples of combination modalities in chemotherapy (Corrie, 2008). However, a narrow therapeutic window due to severe off-target toxicity and lack of target specificity is a major drawback. To overcome the shortcomings of chemotherapeutic agents, drug development for the tumor-associated target based on its biological function has led to the evolution of targeted cancer therapies.
The basis for the development of antibodies for cancer therapy was initially to provide an alternative approach to reduce the undesirable systemic toxicity of chemotherapy; this approach has the advantage of antibody specificity for the tumor antigen to allow killing of the targeted tumor cells. Antibodies have become important therapeutic agents, as evidenced by the growing number of antibody-based drugs listed for US Food and Drug Administration (FDA) approval and for clinical development. The key to successful development of a therapeutic mAb is, in part, the rapid advancement and application of antibody engineering technology (Fig. 1). Currently, multiple approaches for immunotherapy are in development, including the use of unconjugated mAbs, mAb-toxin conjugates (immunotoxin conjugates), mAb-radionuclide conjugates (radioimmunoconjugates), and ADCs. Of these, the significance of therapeutic ADCs, with the emphasis on the emergence of ADC technology and optimization strategies, is highlighted in this review. Other antibody-based immunotherapies have been extensively reviewed elsewhere, and therefore are only briefly described herein, for comparisons with ADC technological development.
A number of unconjugated mAbs have been approved for the treatment of various cancer types (Table 1), and have demonstrated promising clinical benefits. However, additional novel mAbs and improvements of current therapeutic mAbs are needed to further enhance the efficacy as exemplified herein. Rituxan? (Rituximab), a chimeric anti-CD20 antibody, was the first mAb approved by the FDA for NHL. CD20 is the surface marker present in >80% of NHL cases. A profound depletion of circulating B cells followed by complete recovery within a year of initial treatment was observed in most B cell NHL patients treated with Rituxan? (Maloney
The next major advancement in immunotherapy following the use of unconjugated mAbs was arming the antibody with toxic molecules, such as diphtheria toxin and radionuclides (Moolten and Cooperband, 1970; Steiner and Neri, 2011). The concept of antibody-mediated delivery of radionuclides to the tumor site, using ARCs, hence radioimmune therapy, initially failed in early clinical development due to an inadequate radiation dose delivered to the tumor site to obtain clinically meaningful responses. The considerations for ARC development, including the choice of antibody and radionuclide, have been discussed elsewhere (Koppe
There are only two FDA-approved ARCs, Zevalin?, and Bexxar?, both of which were approved in early 2000, and both of which are conjugated to β-emitting radionuclides, 90Y, and 131I, respectively. Although both Zevalin? and Bexxar? utilize a murine-derived antibody, there are no other successful Zevalin? and Bexxar? antibodies (e.g., humanized or human mAb backbone), or other ARCs with FDA approval, despite the evidence for greater clinical efficacy compared to the unconjugated mAb (Morschhauser
Since unconjugated mAbs possess modest antitumor efficacy as single agents, combination therapy with the mAb and a chemotherapeutic drug is now a routine clinical practice to achieve higher therapeutic efficacy. However, the off-target systemic toxicity of chemotherapy remains as a challenge. Therefore, an alternative approach to improve efficacy of mAb therapy, while integrating targeted selectivity of the chemotherapeutic drug, is to conjugate the mAb with a cytotoxic agent via a linker, known as an ADC (Fig. 2). Potent cytotoxic drug delivery of ADCs to tumors, with targeted specificity, would improve the therapeutic efficacy. However, because ADCs are drugs or potential drug candidates, in this review the cytotoxic molecule conjugated to the mAb will be referred to as a payload rather than a drug, unless the payload is the therapeutic drug, itself.
The first generation of ADCs was a mAb conjugated with an anticancer chemotherapeutic drug such as doxorubicin, because ADCs were designed to deliver cytotoxic agents to the specific tumor cells via tumor-specific antigens on the cancer cells (Yang and Reisfeld, 1988; Petersen
Insufficient clinical benefits from the early ADCs were due to their lack of the core attributes for efficacious ADCs, which we now have a better understanding of the required potency of the cytotoxic agents, efficient internalization and stability of ADCs, and the microenvironment of the target antigen and the tumor. Some of the key drawbacks and historical significance of early preclinical and clinical studies of therapeutic ADC development are discussed below.
ADCs with murine-derived antibody backbones were evaluated in clinical trials, but were soon discontinued due to an immune response involving development of human anti-murine antibodies (HAMA) in patients (Petersen
BR96-Dox, an anti-Lewisy mAb conjugated to doxorubicin via a hydrazone linker, failed in Phase II trials for metastatic breast cancer due to low potency of the doxorubicin as the payload and to the instability of the linker. Only 33% of the patients treated with BR96-Dox showed objective responses, despite high antitumor potency in preclinical studies (Trail
Gemtuzumab ozogamacin (Mylotarg?) is considered a second generation ADC; but it was the first generation ADC drug to reach the market. Mylotarg? is an anti-CD33 mAb conjugated to calicheamicin as the payload via an acid-labile hydrazone linker. Mylotarg? was given accelerated approval for treatment of acute myeloid leukemia (AML) during the first relapse of patients >60 years of age (Bross
The clinical development of Mylotarg? involved humanization of murine P67.6 antibody and linker optimization.
During therapeutic ADC development, most efforts involved the optimization of antibody, payload, and linker components of the ADC, which can be readily evaluated and optimized. However, the inherent features of the target are more difficult to address. Consequently, target selection remains one of the critical factors, especially in ADC development. Some of the principles and criteria that should be considered for the selection of good therapeutic ADC targets are discussed below.
Ideal ADCs targeting tumor-specific antigens are those that are exclusively and abundantly expressed on tumor cells and seldom expressed on normal cells. However, potential immunotherapeutic targets are often expressed on both tumor and normal cells. Thus, this is one of the most important criteria to be considered, as the level of target expression will ultimately dictate the therapeutic efficacy of ADCs. Since the antitumor activity of the ADC begins with binding to the target, followed by internalization into the tumor cells, higher expression levels of the target will result in more ADC localized on the tumor cells (Fig. 2). This ultimately results in higher intracellular concentration of the payload, which should enable more effective killing of the tumor cells. Considering only a fraction of administered antibody-based drugs are accumulated in the tumor, high target expression is therefore critically important (Scott
Internalization of ADC upon binding to the target is often necessary for optimal efficacy of the ADC, because cytotoxic payloads typically act on intracellular targets. However, internalization of target antigen, alone, does not appear to be a prerequisite for ADCs to function. Non-internalizing or insufficiently internalizing antigens, such as alternatively spliced extra domains A and B of fibronectin and CD20, were successfully targeted by ADCs in preclinical
Another characteristic of the antigens that may also reduce the binding of ADCs to the targets is the shedding or secreting of antigens, leading to a potentially higher risk of toxicity. Additionally, targets associated with non-solid tumors are expected to have better clinical responses to ADCs than do the solid tumors. Indeed, the first two FDA-approved ADCs, Mylotarg? and Adectris?, are approved for non-solid tumors. However, antigen shedding and tumor type are not absolute limiting factors. As one example, the expression levels of targets and internalization of the ADCs are exemplified by HER2 that is targeted by Kadcyla?. Only about 20% of breast cancer patients are HER2 positive, and soluble HER2 is systemically measurable and represents the target expressed on solid tumors (Wong, 1999; Gajria and Chandarlapaty, 2011). In addition, shedding of CD30 from HL-derived L540 cells was reported as an indication of disease activity, but was successfully targeted by Adectris? (Horn-Lohrens
Strategies for optimization of therapeutic mAbs, such as increasing specificity, affinity, and pharmacokinetics (PK) can be applied to therapeutic ADC development. The tools for generation of more potent therapeutic antibodies have been extensively reviewed elsewhere, and are not described herein. This section is focused on the features of an antibody as a components of the ADC.
ADCs currently in development are comprised of the complete IgG antibody, which is likely optimal due to the favorable PK properties when compared to antibody fragments. Most ADCs on the market and in clinical development are the IgG1 isotype. Only a few of the ADCs in development are IgG2 or IgG4, as is AGS-16M8F (anti-ENPP3 IgG2-MMAF) and inotuzumab ozogamicin (anti-CD22 IgG4-calicheamicin), respectively. However, a systematic comparison of antitumor efficacy for a panel of anti-CD70 antibodies of various IgG isotypes conjugated to a monomethyl auristatin phenylalanine (MMAF) payload demonstrated comparable
Antibodies having effector functions supported by IgG1 and bisecting N-glycosylation can further enhance the efficacy of the ADC. However, the efficacy of the ADC is less significant than the payload delivered by the ADC. Anti-CD70, the antibody component for SGN-70A ADC, has antibody-dependent cell-mediated cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), and complement-dependent cytotoxicity (CDC) functions. Nonetheless, equivalent efficacy among the anti-CD70-MMAF conjugates with IgG1, IgG1v lacking FcγR binding, and IgG2 isotypes was reported (McDonagh
Binding specificity and affinity of ADCs for the targets are additional critical factors to efficacious immunotherapy. However, efforts to engineer antibodies for ADCs have been directed toward control of the conjugation site and for the stoichiometry of the payload for generation of homogenous ADC products. Detailed examples of antibody engineering for such approaches, including insertion of natural and artificial amino acids, are described below (under the
Internalization is critical for ADCs to exert cytotoxic functions on tumor cells. Therefore, failure of ADCs to internalize will result in poor efficiency of payload release, and thus, low efficacy. A recent report on the enhancement of internalization of cetuximab and a cetuximab-drug conjugate resulting in improved therapeutic efficacy further supports the importance of ADC internalization (Chen
ADCs developed to date rely on the internalization of the ADC, and release of active cytotoxic molecules inside the tumor cell. Since expression levels of target antigens on the tumor cells are often limited, the inherent potency of the payload must be sufficient to kill the tumor cell, even at low concentrations. Consequently, an ideal payload for ADCs should have
The highly potent antimitotic compound, dolastain 10, was discovered from the sea hare
Maytansine, isolated from the shrub
Chemical compounds that target DNA are also used as payloads for ADCs. Calicheamicin from
Duocarmycins, isolated from
MDX-1203 is an anti-CD70 conjugated to the duocarmycin derivative MED-2460 via a cleavable valine-citrulline linker. This ADC utilizes a multilayered mechanism for the activation of cytotoxic payloads to maximize the therapeutic index. Following internalization of MDX-1203 into targeted cells, the prodrug MED-2460 is released from the antibody by cleavage of the linker via a lysosomal protease; the prodrug MED-2460 is then activated by carboxyl esterase to form an “active drug” form of MED-2460. This activated MED-2460 cytotoxic payload then alkylates AT-rich regions in the minor groove of DNA. The Phase I clinical evaluation in patients with clear cell renal cell carcinoma (ccRCC) or B cell NHL has been completed. However, the study results and the stability of this multilayered MED-2460 payload have not been reported.
Amatoxins are bicyclic octapeptides found in poisonous mushrooms such as the green death cap mushroom
In addition to the aforementioned cytotoxic payloads, other molecules being investigated include derivatives of pyrrolbenzodiazepines [(PBDs) SGD-1882], doxorubicin, and centanamycin (indolecarboxamide), which all bind to DNA and either alkylate or intercalate into the DNA (Fig. 3) (Beck
Additional cell killing via the bystander effect of membrane-permeable payloads (e.g., MMAE and PBD) compared to the less membrane-permeable payloads (e.g., MMAF) was reported (Li
One of the fundamental lessons learned from the first generation of ADCs is that a suitably stable linker is as vital as the antibody and payload for maximization of therapeutic efficacy of the ADC. The ideal linker is systemically stable so that biophysiochemcial property of ADC are similar to that of the unconjugated antibody, but are still able to release the payload at the target site. Extensive research is being conducted to develop novel linkers for ADCs, and the most broadly evaluated and utilized linker platforms include both cleavable and non-cleavable linkers.
The cleavable linkers include chemically-labile (e.g., hydrazones and disulfides) and protease-labile linkers. These cleavable linkers are designed to be stable in circulation, but release the toxic payloads due to differences between the extracellular and intracellular microenvironment following internalization of the ADC. For example, the acid-labile hydrazone linker of Mylotarg? liberates calicheamicin when encountering an acidic pH environment such as found in lysosomes and endosomes (pH 4∼6) (van Der Velden
The peptide-based linkers are also designed to be retained in the ADC formin circulation, but to release their payload upon cleavage by specific intracellular proteases. For example, Adcetris? uses the valine-citrulline (vc) dipeptide linker, which is hydrolyzed by cysteine protease cathepsin B in lysosomes following endocytosis (Doronina
Other optimization strategies for protease sensitive linkers include use of the β-glucuronide linker, which is recognized and hydrolyzed by β-glucuronidase for payload release (Jeffrey
In contrast to the cleavable linkers, non-cleavable linkers that possess potent antitumor activity were unexpectedly discovered. Non-cleavable thioether and maleimidocaproyl (mc) linkers were initially synthesized for use as controls for the evaluation of cleavable linker conjugates. However, ADCs linked with these non-cleavable linkers, such as huC242-MCC-DM1 and cAC10-L4-MMAF, were as active as the conjugates with the cleavable linkers (Doronina
Some ADCs with non-cleavable linkers showed
Linker optimization has extended to the emergence of a “tunable” linker for payload conjugation at the linker site, rather than at the antibody. Novel SpaceLink technology developed by Syntarga (acquired by Synthon in 2011) utilizes highly flexible linkers, such that the linkers can reversibly attach the payload to the antibody in a modular fashion. This, in turn, enables selection and optimization of the payload and linker-payload combination to generate ADCs with maximal therapeutic potential for the target. SpaceLink technology uses unique linker chemistry to conjugate payloads via hydroxyl groups, and cleavage of the linker triggers spontaneous release of the payloads (De Groot
Perhaps less intuitive in ADC development is the prediction of the optimal linker-payload combination to achieve the most efficacious and tolerable ADC for given targets. Therefore, a throughput systematic approach for the linker and payload selection that will minimize optimization would further advance ADC development. One potential method is the use of radiolabeled linkers and payloads for ADCs, which may help identify the metabolites and free payloads in the circulation using xenograft models (Kitson
Homogeneous ADC production may become a prerequisite for FDA approval for future ADC development and use. It has been shown that optimal drug attachment for ADCs is 2∼4 DARs for favorable efficacy with PK profiles comparable to that of the corresponding unconjugated mAbs (Hamblett
Typically, a cytotoxic molecule is attached to the antibody via alkylation of cysteine or acylation of lysine on the mAb through “controlled” but “random” conjugation reactions, which produces a mixture of ADCs. Modification at lysine is less preferable than cysteine, due to the greater number of lysine residues on mAbs that are solvent accessible for conjugation. Conjugation at cysteine following partial reduction of interchain disulfide bonds also produces heterogeneous ADCs. Thus, antibody engineering has been extended for controlled conjugation reactions to enable the production of homogeneous ADCs with defined sites and stoichiometric drug loading. Both insertion and deletion of cysteine residues in the mAb backbone have been approached to improve the homogeneity of ADCs, as used in anti-MUC16, anti-HER2, anti-CD70, anti-CD33, and anti-CD30 conjugates (McDonagh
Other recombinant technologies employed for antibody engineering were the insertion of unusual amino acids such as selenocysteine (Se-Cys) and acetyl phenylalanine into the antibody backbone for site-specific conjugation. Se-Cys is a bio-orthogonal analog of cysteine with a selenol group in place of the thiol group. Utilization of engineered Se-Cys for site-specific conjugation of mAbs and Fab fragments has been recently demonstrated using rituximab as a prototype (Hofer
Other novel approaches investigated for the controlled conjugation of payloads to antibodies included the chemo-enzymatic bioconjugation approaches, using enzymes, such as glycosyltransferase, transglutaminase, and formyl glycine generating enzyme (FGE). The catalytic activity of mutant galactosyltransferase (1,4Gal-T1-Y289L) for the transfer of activated C2-keto-Gal glycan to the glycosylation site at Asn-297 of mAbs has been reported previously (Ramakrishnan and Qasba, 2002; Boeggeman
Other chemo-enzymatic bioconjugation methods include the use of glutamine and aldehyde tag inserts. Use of trans-glutaminase for ADC generation utilizes the advantage that the enzyme does not recognize the naturally occurring glutamine residues, but recognizes glutamine in the glutamine tag (LLQG) located in a flexible region (Jeger
Various techniques for the incorporation of functional groups into proteins for controlled drug conjugation have been developed as discussed in this section. Some of these conjugation methods resulted in improvement of homogeneity, PK profiles, efficacy, and greater tolerability, and resulted in additional improvements in ADC production. However, further investigations are needed to evaluate the generality and scalability of the conjugation technology, and to compare the efficacy and tolerability of ADCs prepared using different conjugation methods for identification and standardization of payload attachments.
Advancement of ADC core technology development has led to their approval and strategically designed ADCs, including site- and DAR-specific ADCs, are currently in clinical and preclinical developmental stages (Table 2). Adcetris?, an anti-CD30-vcMMAE conjugate with ∼4 DAR was approved to treat HL and relapsed systemic ALCL. Adcetris? is the first approved drug in over 30 years for HL treatment. CD30 is abundantly and selectively expressed on HL and Reed-Sternberg cells, while its expression is highly restricted to activated B and T lymphocytes and natural killer (NK) cells (Deutsch
The most advanced of the ADC drug candidates in the clinic, that has yet to be launched, is inotuzumab ozogamicin (CMC-544), an anti-CD22-calicheamicin conjugate in Phase 3 for relapsed or refractory CD22-positive acute lymphoblastic leukemia (ALL). CD22 is a cell surface sialoglycoprotein expressed on over 90% of leukemic lymphoblasts in a majority of B-lineage ALL patients. Remarkable clinical response rates from Phase I/II studies were observed for CD22-positive ALL patients treated with CMC-544 (Leonard
Most ADCs in clinical use, including Adcetris? and Kadcyla?, are heterogeneous ADCs that differ in drug conjugation sites and DAR number. However, the most advanced stage for site- and DAR-specific ADC in clinical use is polatuzumab vedotin (DCDS4501A), indicated for diffuse large B cell lymphoma (DLBCL) and NHL (Dornan
ADCs with recently emerging payloads are SGN-CD33A and SGN-70A, both of which use a PBD dimer cytotoxic agent, and are currently in Phase I trials for the treatment of AML (for SGN-33A), and NHL and renal cell carcinoma (for SGN-70A). It is important to note that these ADCs have the PBD dimer conjugated to the engineered cysteine (S239C) of the antibody via a protease-cleavable maleimidocaproyl-valine-alanine dipeptide linker for homogenous ADC products with a 2 DAR specification (Kung Sutherland
Additional ongoing clinical studies of ADCs include CD19-targeting ADCs, including SAR3419 and SGN-19A, both of which appear to have a similar MTD and therapeutic potential as the newer drug candidates. SAR3419 is an anti-CD19-DM4 conjugate under development by Sanofi using ImmunoGen ADC technology. Although SAR3419 for ALL has been discontinued due to lack of therapeutic efficacy compared to its competitors (Sanofi, 2014), promising clinical results against DLBCL were observed. The Phase II STARLYTE (Clinical-Trials.gov #NCT01472887) trial of SAR3419 showed >40% response rate as a single agent in patients with relapsed or relapsed/refractory CD19-positive DLBCL, and among the responding patients whom had not responded to first line treatment (Trneny
Extensive research focused on each component of the ADC that contributes to successful therapeutic ADC development, and a more informed selection of ADC target strategies have led to the approval of ADCs and increases in ADCs in the pipeline (Table 2). The addition of inotuzumab ozogamicin for ALL indications and the promising clinical data for extension of Adectris? in additional therapeutic indications are anticipated sometime later this year. The evolution of cancer therapy from targeted unconjugated mAbs to ADC for better clinical outcomes will drive the development of the next generation of immunotherapies for cancer.
However, discontinuation of some ADCs in clinical development (Table 3) may also occur due to insufficient clinical efficacy or safety concerns related to the payload toxicology. Inotuzumab ozogamicin, for example, was recently discontinued for NHL indication in Phase III trials due to lack of clinical efficacy that did not correlate well with the preclinical
Homogeneous ADC products are likely required to obtain FDA approval in the near future. And such homogeneous products are also desired by ADC drug manufacturers since better PKs and safety profiles are anticipated with reduction in undesirable higher DAR mixtures in the ADC product. Consequently, the technological development in site-directed conjugation chemistry, along with antibody engineering, will continue to emerge for the development of homogenous ADCs; these are the gold standard attributes for conjugated biological drugs. Future research must improve the solubility of the payload or the linker to mask payload hydrophobicity and thus improve the current site-directed conjugation technology to further enhance physiobiochemical and PK stability of ADCs (Zhao
Recently emerging cytotoxic payloads, including the PBD dimer and α-amanitin, targeting at the DNA and RNA levels, respectively, have demonstrated clinical or preclinical anti-tumor efficacy. Likewise, the development of novel cytotoxic payloads with different cellular targets and metabolic processes could be additional areas of focus to improve clinical responses and broaden therapeutic options for cancer treatment. In particular, greater opportunities and challenges exist for the development of payloads and linkers that are non-substrates for drug-efflux pumps to bypass MDR resistance for cancer therapies. Clinically proven payloads for ADCs (e.g., calicheamicin, MMAE, and DM1) are the substrates for MDR. Recently developed ADCs with a PBD dimer payload (e.g., SGN-CD33A) or with PEGylated linkers (e.g., anti-EpCAM-PEG4Mal-DM1) generated metabolites that were efficacious in MDR-expressing tumor cells, further demonstrating the potential enhancement of the therapeutic window for ADCs.
Perhaps the most intriguing preclinical developments of anticipated ADCs are the bispecific antibody-drug conjugates (BDCs) and antibody fragment drug conjugates (FDCs). Blina-tumomab (Blincyto?) is the first bispecific anti-CD19 and anti-CD3 mAbs approved by the FDA in 2014 for ALL, and ∼20 bispecific mAbs are currently in clinical development. Cytotoxic drugs conjugated to the antibodies that target two tumor-specific antigens could provide better efficacy and safety, which in turn, would increase the therapeutic index above the corresponding conventional monospecific ADCs or the unconjugated bispecific antibodies. Thus, extension of ADC technology into bispecific antibody use for BDC development could provide further ADC optimization. The challenges associated with the identification of two targets that are preferentially expressed on the same tumor or in the microenvironment, that favors bispecificity and production of homogenous BDCs, must first be overcome to gain popularity.
FDC is an alternative ADC platform that may be developed again in the future. Antibody fragments such as diabody and Fabs were investigated in the past to improve tumor penetration.
The lessons learned from both unpromising and successful ADCs, along with the continued emergence of diverse ADC core technologies, will make future ADC development more successful for cancer treatments. The strategic design of effective ADCs for the treatment of other conditions, such as autoimmune diseases, could result from the current clinical trials, as some of the chemotherapeutic drugs, such as methotrexate and cyclophosphamide, are already used in diseases other than cancer. The improvement of the therapeutic window, elucidation of the ADC mechanism of action, and decrease of off-target toxicities remain as challenges for future ADC development.
We thank Dr. Sung Ho Woo and Hyuck Choi for assistance with the Fig. 3. This work was supported by NRF-2011-0025320 and by the Ministry of Trade, Industry, and Energy (10047748).