The antibody component of ADCs serves as the targeting moiety, providing tumor selectivity through antigen recognition. Modern ADCs predominantly utilize humanized or fully human IgG1 antibodies to minimize immunogenicity while maintaining effector functions [¹⁴,¹⁵,¹]. Critical parameters for antibody selection include high binding affinity to tumor-associated antigens, efficient internalization kinetics, and favorable pharmacokinetic properties, including prolonged serum half-life [¹⁶,¹⁷]. The shift from murine to humanized antibodies has substantially reduced immunogenicity-related complications that plagued early ADC development [¹⁴,¹⁸]. Target antigen selection represents a pivotal decision in ADC design. Ideal targets exhibit high expression on tumor cells with minimal expression on normal tissues, undergo efficient receptor-mediated endocytosis, and demonstrate limited antigen shedding [¹,¹⁹]. Current approved ADCs target oncoproteins such as HER2, EGFR, and HER3, as well as tissue-associated antigens including TROP2, Nectin-4, and tissue factor [¹²,²⁰,²,²¹]. In HER2-positive breast cancer specifically, the DESTINY-Breast03 trial demonstrated that trastuzumab deruxtecan achieved superior progression-free survival (median 28.8 versus 6.8 months) compared to trastuzumab emtansine [⁵]. Linker Chemistry and Payload Release Linker technology plays a critical role in determining ADC stability, payload release kinetics, and the potential for bystander effects. Two main categories exist: cleavable linkers that respond to tumor-specific conditions (protease activity, acidic pH, or glutathione levels), and non-cleavable linkers that require complete antibody degradation for payload release [¹⁴,¹,²²,²³]. In preclinical models, cleavable linkers enable bystander killing of neighboring antigen-negative cells, potentially beneficial in heterogeneous solid tumors, though clinical data directly demonstrating superior efficacy from bystander effects in patients remain limited [²⁴,²⁵]. Recent innovations include hydrophilic linkers that improve aqueous solubility and reduce aggregation in manufacturing (demonstrated in vitro), though clinical impact on pharmacokinetics requires further study [¹⁴,²⁶]. The drug-to-antibody ratio (DAR) significantly impacts ADC pharmacology and therapeutic index. Traditional conjugation methods produced heterogeneous mixtures with DARs of 0-8, leading to variable efficacy and toxicity [¹⁴,²⁷,²⁸]. Site-specific conjugation technologies have emerged to generate more homogeneous ADCs with precise DAR control [²⁹,³⁰]. However, clinical data demonstrate that optimal DAR varies by payload class and target biology [²⁷,²⁸]. For instance, deruxtecan-based ADCs utilize high DARs of approximately 8 to maximize payload delivery, whereas auristatin-based ADCs typically employ DARs of 3-4 to balance efficacy and tolerability [³¹,³²,²⁴].

Payload selection represents a defining characteristic of ADC design, with different cytotoxic classes offering distinct mechanisms and therapeutic profiles. The three predominant payload categories are microtubule inhibitors (auristatins and maytansinoids), DNA-damaging agents (calicheamicins and pyrrolobenzodiazepines), and topoisomerase inhibitors (deruxtecan and exatecans) [³¹,³³,³⁴,³⁵]. In comparative analyses of clinical trial data, topoisomerase inhibitor payloads have demonstrated favorable efficacy profiles across multiple solid tumor types [³⁶,³⁷]. The required potency for ADC payloads exceeds that of conventional chemotherapy by orders of magnitude, with effective concentrations in the picomolar to nanomolar range [³⁸,³⁵]. This exceptional potency enables tumor cell killing despite the limited number of antibody molecules that successfully bind and enter tumor cells. Payload selection must balance cytotoxic potency with acceptable systemic toxicity when the ADC is catabolized or when free drug is released prematurely in circulation [³¹,³⁹,³⁵].

Trastuzumab deruxtecan has demonstrated transformative efficacy in HER2-positive breast cancer. The pivotal DESTINY-Breast03 trial, comparing trastuzumab deruxtecan to trastuzumab emtansine in previously treated HER2-positive metastatic breast cancer, showed median progression-free survival of 28.8 versus 6.8 months (hazard ratio 0.28, 95% confidence interval 0.22-0.37), with objective response rates of 79.7% versus 34.2%.4 This represented a practice-changing advancement, establishing trastuzumab deruxtecan as the preferred second-line therapy [³⁷,⁴⁰]. Beyond HER2-positive disease, trastuzumab deruxtecan has demonstrated activity in HER2-low breast cancer (immunohistochemistry 1+ or immunohistochemistry 2+/in situ hybridization-negative), a population previously considered HER2-negative and ineligible for HER2-targeted therapy. The DESTINY-Breast04 trial enrolled patients with HER2-low metastatic breast cancer who had received prior chemotherapy, demonstrating median progression-free survival of 9.9 months versus 5.1 months with physician’s choice chemotherapy (hazard ratio 0.50) and median overall survival of 23.4 months versus 16.8 months (hazard ratio 0.64) [⁷]. FDA approval followed in 2022, expanding the population eligible for HER2-targeted ADC therapy [²¹]. TROP2-Targeted ADCs Sacituzumab govitecan, targeting TROP2 (trophoblast cell-surface antigen 2), has demonstrated efficacy across multiple breast cancer subtypes. In metastatic triple-negative breast cancer, the ASCENT trial showed median progression-free survival of 5.6 versus 1.7 months with chemotherapy (hazard ratio 0.41) and median overall survival of 12.1 versus 6.7 months (hazard ratio 0.48), leading to FDA approval in 2020 [⁴]. Subsequently, the TROPiCS-02 trial evaluated sacituzumab govitecan in hormone receptor-positive, HER2-negative metastatic breast cancer, demonstrating median progression-free survival of 5.5 versus 4.0 months (hazard ratio 0.66) and median overall survival of 14.4 versus 11.2 months (hazard ratio 0.79) [¹⁰]. FDA approval for this indication followed in 2023. The mechanism involves delivery of SN-38, the active metabolite of irinotecan, to tumor cells expressing TROP2 [¹²].

Enfortumab vedotin, targeting Nectin-4, has established efficacy in advanced urothelial carcinoma. The EV-301 trial demonstrated superior overall survival with enfortumab vedotin versus chemotherapy in previously treated patients (12.9 versus 8.9 months, hazard ratio 0.70), leading to FDA approval in 2019 [⁸]. More recently, the EV-302 trial demonstrated that enfortumab vedotin plus pembrolizumab achieved superior progression-free survival (12.5 versus 6.3 months, hazard ratio 0.45) and overall survival (31.5 versus 16.1 months, hazard ratio 0.47) compared to platinum-based chemotherapy in the first-line treatment of locally advanced or metastatic urothelial carcinoma, establishing this combination as a new standard of care [⁹]. Tisotumab vedotin, targeting tissue factor, received FDA approval in 2021 for recurrent or metastatic cervical cancer following the innovaTV 204/GOG-3023/ENGOT-cx6 trial, which demonstrated an objective response rate of 24% in heavily pretreated patients [⁴¹]. This represented an important advancement in a disease with limited treatment options.

Trastuzumab deruxtecan has demonstrated efficacy beyond breast cancer. In HER2-positive gastric or gastroesophageal junction cancer, the DESTINY-Gastric01 trial showed an objective response rate of 51% in patients previously treated with trastuzumab-based therapy, with a median overall survival of 12.5 months [¹¹]. FDA approval for this indication was granted in 2021. In HER2-mutant non-small cell lung cancer, the DESTINY-Lung01 trial demonstrated an objective response rate of 55% with trastuzumab deruxtecan in patients who had received prior platinum-based chemotherapy [⁶]. FDA approval followed in 2022. More recently, datopotamab deruxtecan targeting TROP2 in non-small cell lung cancer showed activity in the TROPION-Lung01 trial, though the study did not meet its co-primary endpoint in the overall population; as of late 2025, regulatory review is ongoing [⁶,⁴²].

Several novel targets are under active clinical investigation. B7H3 (CD276), expressed across multiple solid tumor types, is being targeted by investigational ADCs in early-phase trials [⁴³,⁴⁴]. CEACAM5, overexpressed in colorectal and other gastrointestinal malignancies, represents another emerging target [⁴³]MET, c-Kit, and CD70 are additional targets under evaluation, though clinical data remain preliminary [⁴³,¹⁹]. Folate receptor alpha has shown promise with mirvetuximab soravtansine in platinum-resistant ovarian cancer [⁴⁵]

Interstitial lung disease represents a serious toxicity associated with deruxtecan-based ADCs (trastuzumab deruxtecan, datopotamab deruxtecan). Across trastuzumab deruxtecan trials, ILD occurs in 10-15% of patients, with grade 3 or higher events in 2-4% and rare fatal cases reported [⁵,³²,³]. The mechanism is not fully elucidated but may involve payload-mediated lung toxicity. Management requires baseline chest imaging, patient education on respiratory symptoms, prompt evaluation of any new or worsening dyspnea or cough, and immediate discontinuation of any-grade ILD.2 Prophylactic corticosteroids are not recommended, but corticosteroids are used for treatment of confirmed ILD. Patients with pre-existing ILD or impaired pulmonary function require careful risk-benefit assessment before initiating deruxtecan-based therapy.

Neutropenia is the most common dose-limiting toxicity for sacituzumab govitecan, occurring in approximately 51% of patients (grade 3 or higher in 27%) [⁴,¹²,¹⁰]. The SN-38 payload (the active metabolite of irinotecan) undergoes hepatic glucuronidation, and patients with the UGT1A1*28 polymorphism (reduced glucuronidation capacity) are at higher risk of severe neutropenia [⁴]. Growth factor support per institutional guidelines is recommended for grade 3-4 neutropenia, with dose reductions for recurrent toxicity. Other ADCs demonstrate lower rates of severe neutropenia, though grade 1-2 cytopenia is common across agents [¹³].

Peripheral neuropathy is common with microtubule inhibitor-based ADCs, particularly those using monomethyl auristatin E payloads. Enfortumab vedotin causes peripheral neuropathy in approximately 50% of patients, with grade 3 or higher events in 5%.7,8,49,56 Management requires dose modification: hold for grade 2 neuropathy until improvement to grade 1 or lower, reduce dose for recurrent grade 2, and discontinue for grade 3-4 [⁸]. The cumulative nature of neuropathy necessitates ongoing assessment throughout treatment. Maytansinoid-based ADCs also cause neuropathy, though generally at lower rates than monomethyl auristatin E-based agents [¹⁸,⁴⁶].

Tisotumab vedotin, targeting tissue factor, causes ocular adverse events in the majority of patients due to tissue factor expression in ocular tissues. Management requires prophylaxis before each infusion with ocular lubricants, ophthalmic corticosteroids, and vasoconstrictor eye drops, along with regular ophthalmologic examinations [⁴¹]. Despite prophylaxis, ocular toxicity, including conjunctivitis, dry eye, and vision changes, remains common. Patients should be counseled on the importance of adherence to prophylactic measures.

Additional toxicities include diarrhea (particularly with SN-38-containing ADCs like sacituzumab govitecan), dermatologic reactions (common with deruxtecan-based ADCs), hepatotoxicity (generally low-grade across agents), and infusion reactions (typically grade 1-2 and manageable with premedication) [¹³,⁴⁷,³]. Agent-specific product labels and consensus management guidelines should guide toxicity monitoring and intervention [⁴⁸].

Downregulation or loss of target antigen expression represents a primary resistance mechanism. In HER2-targeted ADCs, clinical observations demonstrate that some patients progress with preserved HER2 expression, while others show HER2 loss at progression [⁴⁹,¹⁴,⁵¹]. Antigen heterogeneity within tumors may allow selection of antigen-low or antigen-negative clones during therapy [¹⁹,⁵²]. For trastuzumab deruxtecan, studies of HER2-low breast cancer suggest that even low antigen expression may be sufficient for efficacy, potentially reducing antigen loss as a resistance mechanism compared to trastuzumab emtansine [¹⁴,⁷,²⁴]. Impaired antigen internalization represents another antigen-related mechanism. ADCs require receptor-mediated endocytosis to deliver payload intracellularly. Defects in endocytic machinery, including mutations or downregulation of proteins involved in clathrin-mediated endocytosis, can impair ADC internalization and reduce cytotoxicity [¹⁴,⁵³,⁵¹,¹⁷]. In preclinical models of trastuzumab emtansine resistance, trafficking defects leading to reduced lysosomal delivery have been identified [⁵³,⁵¹].

Resistance mechanisms vary by payload class. For topoisomerase I inhibitor-based ADCs (deruxtecan, exatecans), preclinical data suggest that TOP1 mutations reducing enzyme-DNA complex formation may confer resistance, similar to resistance patterns observed with conventional topoisomerase inhibitors [⁴³,³¹]. However, clinical data confirming TOP1 mutations as a clinically relevant resistance mechanism in patients treated with deruxtecan-based ADCs are limited. For microtubule inhibitor-based ADCs (auristatins, maytansinoids), resistance mechanisms include tubulin mutations that reduce drug binding and alterations in microtubule dynamics [³¹,³³,⁵¹,¹⁸]. Upregulation of multidrug resistance transporters, particularly P-glycoprotein (MDR1/ABCB1) and breast cancer resistance protein (BCRP/ABCG2), can increase payload efflux and reduce intracellular accumulation [¹⁴,⁵¹,⁵⁴]. The impact of efflux pump expression may vary depending on the payload’s lipophilicity and whether the linker is cleavable [⁵⁴].

The tumor microenvironment influences ADC efficacy through multiple mechanisms. Elevated interstitial fluid pressure, dense extracellular matrix, and abnormal vasculature in solid tumors can impair ADC penetration and distribution [⁵⁵,¹⁷]. These physical barriers may be particularly relevant for large ADC molecules (molecular weight approximately 150 kilodaltons) compared with small-molecule chemotherapy [⁵⁵]. Additionally, tumor-associated macrophages and other stromal cells may internalize ADCs via Fc-gamma receptors, reducing the fraction of ADC reaching tumor cells [³⁹,²]. Alterations in the lysosomal environment, including changes in pH or protease activity, may affect payload release from ADCs utilizing pH-sensitive or protease-cleavable linkers [¹⁴,⁵³].

Potential strategies to address ADC resistance include sequential therapy with ADCs targeting different antigens or utilizing different payload classes, combination approaches with agents that modulate resistance pathways (e.g., efflux pump inhibitors), and development of next-generation ADCs with enhanced tissue penetration or dual-target specificity [⁴³,¹⁹,⁵⁰,²]. Biomarker development to predict resistance and guide therapeutic decisions remains an area of active investigation [⁵⁶].

Bispecific ADCs, which bind two different antigens simultaneously, represent a strategy to address tumor heterogeneity and antigen-mediated resistance [¹⁴,⁵⁷,⁴³]. By targeting two antigens expressed on tumor cells, bispecific ADCs may maintain activity even when one antigen is downregulated. Additionally, bispecific formats can enhance tumor specificity by requiring co-expression of both antigens for optimal binding and internalization [⁵⁷]. Several bispecific ADC formats are in preclinical and early clinical development [⁴³,⁴⁴].

ADCs conjugated to two different cytotoxic payloads offer potential advantages, including simultaneous targeting of multiple cellular pathways and potential payload synergy [⁴³,⁵⁸,⁵⁹]. This approach may delay or prevent resistance by requiring tumor cells to develop resistance mechanisms against two distinct payloads. However, the complexity of manufacturing dual-payload ADCs and optimizing the ratio of each payload presents technical challenges [⁵⁸].

Immune-stimulating antibody conjugates deliver immunomodulatory payloads such as Toll-like receptor agonists or stimulator of interferon genes agonists to the tumor microenvironment, aiming to activate anti-tumor immunity while maintaining tumor-targeted delivery [⁶⁰,⁴³,⁶¹]. In preclinical models, immune-stimulating antibody conjugates have demonstrated the ability to induce immunogenic cell death and enhance T cell infiltration. Early clinical trials are evaluating safety and preliminary efficacy. Combination with Immunotherapy Rational combinations of ADCs with immune checkpoint inhibitors are supported by preclinical data suggesting that ADCs may enhance immunogenicity by inducing immunogenic cell death and releasing tumor antigens [⁶⁰,¹⁹,⁶¹]. The EV-302 trial, demonstrating superior outcomes with enfortumab vedotin plus pembrolizumab compared to chemotherapy alone in first-line urothelial carcinoma, provides clinical validation of this approach [⁹]. However, managing overlapping toxicities, optimizing dosing schedules, and identifying predictive biomarkers remain challenges. Multiple trials are evaluating ADC-immunotherapy combinations across tumor types [⁶²,⁶¹].

Combinations of ADCs with other targeted therapies, including cyclin-dependent kinase 4/6 inhibitors, poly (ADP-ribose) polymerase inhibitors, and tyrosine kinase inhibitors, are under investigation based on preclinical synergy [¹⁹,⁵⁶,⁶¹]. Challenges include managing the toxicity of combined regimens and identifying patient populations most likely to benefit. Sequential strategies, in which ADCs are administered at progression following targeted therapy, have also been explored [⁶²].