Antibody-drug conjugates(ADC) are formed by linking monoclonal antibodies targeting specific antigens with small molecule cytotoxic drugs through linkers, combining the powerful killing effect of traditional small molecule chemotherapy with the tumor targeting ability of antibody drugs. ADC consists of three main components: the antibody responsible for selectively recognizing cancer cell surface antigens, the drug payload responsible for killing cancer cells, and the linker connecting the antibody and the payload.
ADC has become a popular class of drugs for treating hematological malignancies and solid tumors, undergoing extensive preclinical and clinical research. However, like most cytotoxic drugs, the duration of objective response or clinical benefit generated by ADC as a monotherapy remains limited due to the emergence of resistance mechanisms. Therefore, the combination of ADC with other anticancer drugs has become an important direction in ADC drug development.
Currently, regulatory agencies have approved combinations of ADC with chemotherapy/chemotherapy-immunotherapy for hematological tumors, and the FDA has granted breakthrough therapy designation to enfortumab vedotin and pembrolizumab. The most attractive drugs to combine with ADCs are those that have additive or synergistic effects on tumor cells or their microenvironment without unacceptable overlapping toxicities. Combination therapies involving anti-angiogenic drugs, HER2-targeted drugs, DNA damage response agents, and immune checkpoint inhibitors (ICIs) are currently active areas of research.
ADC Combination with Chemotherapy
Optimal combinations of ADC with chemotherapy drugs require a better understanding of unique cell cycle interactions and how cytotoxic partners regulate surface antigen expression. So far, increasing amounts of preclinical and clinical data have shown promising applications and provided valuable insights for guiding further drug development.
Cell Cycle Interactions
DNA damaging agents that act on the S phase and cause G2/M phase arrest (such as antimetabolites, platinum, and topoisomerase inhibitors) can be combined with microtubule inhibitors. The successful combinations of carboplatin with mirvetuximab soravtansine, anetumab ravtansine, or luveltamab tazevibulin in ovarian cancer models illustrate this concept. In early clinical trials, ADCs based on ravtansine combined with carboplatin or doxorubicin showed significant efficacy in platinum-sensitive and resistant ovarian cancer patients, as did ADCs based on deruxtecan combined with capecitabine or cisplatin in gastric and lung cancer patients.
Design of Dosing Schedules
Dosing schedules may be related to drug combination design. Microtubule protein polymerization is a key component of the ADC endocytosis mechanism, and DNA damage-mediated G2/M phase arrest may require time for microtubule disruption to sensitize. Studies in colorectal cancer, lung cancer, and breast cancer models have demonstrated this well, showing that sequential dosing of SGN-15 (Lewis Y antigen-doxorubicin) and paclitaxel caused more DNA fragmentation than concurrent dosing. This observation suggests that adjusting dosing schedules, particularly delaying the administration of DNA damaging agents after anti-microtubule drugs, may enhance therapeutic efficacy.
Regulation of Surface Antigens
Chemotherapy drugs can regulate the expression of surface antigens targeted by ADCs. In this regard, gemcitabine has been shown to upregulate HER2 expression in pancreatic adenocarcinoma cells, where the combination of gemcitabine and trastuzumab emtansine exhibits enhanced potency. Intertwined with the aforementioned cell cycle interactions, HER2 upregulation occurs particularly in the G2/M population, resulting from gemcitabine-mediated DNA synthesis inhibition.
Overlapping Toxicities
ADC is essentially chemotherapy; therefore, the efficacy of combination regimens is often hindered by unacceptable toxicities. Major toxicities are driven by the metabolites of cytotoxic payloads, which must be carefully considered when designing combination strategies. These toxicities include peripheral neuropathy caused by MMAE and DM1 derivatives, ocular toxicity caused by MMAF and DM4, gastrointestinal effects of DM1 or topoisomerase inhibitors, or hepatotoxicity caused by calicheamicin derivatives, as well as nearly universal neutropenia and thrombocytopenia.
Two phase 2a/b studies have demonstrated this, investigating trastuzumab emtansine combined with docetaxel or paclitaxel for HER2+ advanced breast cancer, where over half of the patients required dose reductions or discontinuation of paclitaxel. Newer, more tumor-selective ADC drugs, such as mirvetuximab soravtansine and datotomab deruxtecan, show milder toxicities, making them ideal partners for chemotherapy drugs with different mechanisms of action.
ADC Combination with Targeted Drugs
Compared to standard chemotherapy, ADCs have improved therapeutic indices and increased activity against selective tumor populations, making them ideal partners for targeted drugs. Various combination strategies can be envisioned to overcome treatment resistance and clonal heterogeneity, trigger stronger inhibition of oncogene-dependent signaling pathways, increase the availability of surface antigens, sensitize tumors with low antigen expression, and modulate the tumor microenvironment.
Replacing Chemotherapy with ADCs
So far, many studies have attempted to replace standard chemotherapy with ADCs in combination with targeted drugs, but the results have been disappointing. Clinical trials such as KAITLIN, KRISTINE, and MARIANNE were designed based on the synergistic anti-tumor activity of trastuzumab emtansine combined with pertuzumab, yet they did not show enhanced efficacy compared to paclitaxel, trastuzumab, and pertuzumab in neoadjuvant and metastatic settings. Similarly, in ovarian cancer, the combination of bevacizumab and anetumab ravtansine showed lower efficacy than paclitaxel.
Tyrosine Kinase Inhibitors (TKIs)
Dual-target blockade by adding TKIs can provide greater selectivity and potentially improve therapeutic indices. In the TEAL study, the combination of trastuzumab emtansine, pan-HER2 inhibitor lapatinib, and albumin-bound paclitaxel showed improved responses in neoadjuvant treatment of HER2+ breast cancer patients compared to standard paclitaxel, trastuzumab, and pertuzumab. The combination of trastuzumab emtansine and tucatinib (a more selective anti-HER2 TKI) achieved a 47% objective response rate (ORR), including a 36% brain-specific response rate in patients who progressed after prior paclitaxel and trastuzumab treatment. Next-generation ADCs and TKIs may yield better results.
Targeting ADC Resistance
Increasing evidence supports that targeted drugs can simultaneously target known ADC resistance mechanisms. For instance, as HER2-dependent malignant transformation of breast epithelial cells relies on cyclin D1, CDK4/6 inhibitors have been used in combination with trastuzumab emtansine in HER2-resistant patients. Furthermore, another key cell cycle regulator, PLK1, has recently been identified as an upregulated target in acquired and primary trastuzumab emtansine-resistant models, with its inhibitor volasertib re-sensitizing trastuzumab emtansine in vitro and in vivo. On the other hand, ADCs may also be effective combinations for modulating resistance mechanisms of targeted drugs. For example, the combination of osimertinib and trastuzumab emtansine produced additional anti-tumor effects, with trastuzumab emtansine able to delay or overcome osimertinib resistance in EGFR-mutant non-small cell lung cancer models.
Regulation of Surface Antigens
Some TKIs have been shown to regulate surface antigens, potentially promoting further ADC activity and sensitizing tumors with low antigen expression. In this regard, lapatinib, neratinib, tucatinib, and poziotinib have been shown to enhance the efficacy of trastuzumab emtansine. However, the specific mechanistic principles remain unclear. Among them, lapatinib increases HER2 abundance through strong transcriptional upregulation and reduced ubiquitination, while neratinib decreases surface HER2 abundance by stimulating internalization and endocytosis. The effects of tucatinib on cell surface HER2 remain elusive, while poziotinib upregulates exon 20 mutations but does not upregulate wild-type HER2, suggesting that the synergy mechanism is unrelated to surface HER2 density.
Anti-Angiogenesis
Anti-angiogenic agents can facilitate ADC penetration and tumor cell exposure. The combination of anetumab ravtansine or mirvetuximab soravansine with bevacizumab has shown complete response efficacy in preclinical models of ovarian cancer. A recent phase 1b study combined mirvetuximab soravansine and bevacizumab in heavily pretreated, platinum-resistant, FRα-high ovarian cancer patients, where the 39% ORR exceeded the benchmark of the pivotal AURELIA trial (27%).
DNA Damage Response Agents
Developing synthetic lethality by combining drugs targeting the DNA damage response (DDR) with ADCs carrying DNA damaging agents may be a promising strategy for treating genomically unstable tumors.
Traditionally, the combination of DDR drugs with chemotherapy has been hindered by intolerable toxicities, while the superior activity and tolerability of next-generation ADCs carrying topoisomerase I inhibitor payloads make them more suitable as partners. Multiple clinical trials are exploring this strategy, including niraparib and trastuzumab duocarmazine, talazoparib and sacituzumab govitecan, and olaparib and trastuzumab deruxtecan. Besides PARP inhibitors, the selective increase of ADCs in chemotherapy will undoubtedly expand the range of combinable DDR drugs, such as the ongoing clinical trial of ATR inhibitor berzosertib combined with sacituzumab govitecan (NCT04826341).
ADC Combination with Immunotherapy
Combination strategies of immunotherapy and ADC have recently entered clinical trials. Although preclinical data and early clinical research results suggest enhanced anti-tumor activity, randomized clinical trial results supporting this approach over standard treatment are still awaited.
Anti-PD-1/PD-L1 and Anti-CTLA-4 Antibodies
Increasing evidence suggests that ADCs may enhance the efficacy of immunotherapeutic agents. The mechanisms involved are diverse, including enhanced immune memory and expression of immune regulatory proteins (such as PD-L1 and MHC). Some ADCs have shown greater potency in preclinical models with intact immune systems, supporting their immunomodulatory functions.
Various HER2-targeted ADCs, including trastuzumab emtansine, trastuzumab deruxtecan, and disitamab vedotin, have been tested in vitro and in vivo in combination with ICIs, confirming synergistic activity associated with enhanced homing and activation of immune effectors. The KATE2 study is the only published randomized trial examining ADC plus ICI, comparing the efficacy of trastuzumab emtansine combined with atezolizumab versus trastuzumab emtansine combined with placebo in pre-treated HER2+ breast cancer patients. The combination therapy failed to improve progression-free survival (8.2 vs 6.2 months, P=0.33), indicating that adding ICIs to HER2-targeted therapy may only benefit PD-L1 positive populations. Despite the disappointing results observed in this randomized trial, clinical exploration continues across various tumors.
Additionally, increasing preclinical evidence suggests that combination therapies may restore immune sensitivity. For instance, in models of ICI-refractory melanoma and non-small cell lung cancer (NSCLC), AXL-specific ADC enapotamab vedotin was tested in combination with anti-PD-1 antibody, where the ADC enhanced ICI activity by inducing T cell infiltration and enhancing antigen presentation, leading to pro-inflammatory changes in the TME.
ADC Combined with Other Immunotherapies
Polatuzumab vedotin has been shown to enhance CD20 expression on tumor cells by increasing AKT and ERK signaling, supporting its combination with anti-CD20 antibodies (such as rituximab) and CD20/CD3 bispecific antibody therapies. Additionally, combinations of ADCs with immune modulators are also being explored in other diseases, such as multiple myeloma. In preclinical models, the combination of belantamab mafodotin with OX40 agonists produced synergistic anti-tumor activity, increasing the infiltration and activation of T cells and dendritic cells within tumors. Combinations of belantamab mafodotin are actively being researched clinically, such as the DREAMM-5 study (NCT04126200), which is being tested in combination with various immunotherapeutic agents including (anti-ICOS antibodies, OX40 agonists, and γ-secretase inhibitors) as well as anti-PD-1 antibodies. Preliminary results indicate that the combination with anti-ICOS antibodies shows good activity in heavily pretreated patients.
Furthermore, the design of ADCs can target not only cancer cells but also elements of the TME, such as immune cells or fibroblasts, thereby altering immune responsiveness. For example, ADCs targeting CD73 have shown encouraging preclinical activity. Additionally, in preclinical models, ADCs targeting cancer-associated fibroblasts have shown enhanced CD8+ T cell-mediated anti-tumor activity when used in combination with pembrolizumab.
As monotherapy, ADCs have proven anti-tumor efficacy and have been approved for various solid tumors and hematological malignancies. Academia and industry are currently engaged in extensive efforts to develop next-generation ADCs by identifying new targets and enhancing their pharmacological effects, as well as combination therapies based on current ADCs.
However, to date, the success of combination approaches using first- and second-generation ADCs has been limited, which may be attributed to several factors, such as non-specific expression of targets leading to adverse reactions in normal tissues, overlapping toxicities, insufficient efficacy across different tumor clones, and emerging resistance mechanisms. Therefore, a deep understanding of the pharmacology of ADCs and the associated predictive biomarkers is necessary, with preclinical evaluations in well-characterized patient-derived xenograft models to select the most promising ADC-based combinations. It is believed that future ADC-based combination therapies will show bright prospects.
References:
1. Antibody-drug conjugates: in search of partners of choice. Trends Cancer. 2023 Feb 4
Source: Xiao Yao Shuo Yao 2024-03-01