A narrative review of antibody–drug conjugates in EGFR-mutated non-small cell lung cancer

In the past 15 years, non-small cell lung cancer (NSCLC) treatment has changed with the discovery of mutations and the development of new targeted therapies and immune checkpoint inhibitors. Epidermal growth factor receptor (EGFR) was the first mutation in NSCLC to have a drug that was FDA-approved in 2013. Osimertinib, a third-generation tyrosine kinase inhibitor, is approved as first-line therapy for advanced NSCLC and in the adjuvant setting for Stage IB-IIIA resected NSCLC. However, resistance to osimertinib is inevitably an issue, and thus patterns of resistance to EGFR-mutated NSCLC have been studied, including MET amplification, EGFR C797X-acquired mutation, human epidermal growth factor 2 (HER2) amplification, and transformation to small cell and squamous cell lung cancer. Current management for EGFR-mutated NSCLC upon progression of EGFR TKI is limited at this time to chemotherapy and radiation therapy, sometimes in combination with the continuation of osimertinib. Antibody–drug conjugates (ADCs) are made up of a monoclonal antibody linked to a cytotoxic drug and are an increasingly popular class of drug being studied in NSCLC. Trastuzumab deruxtecan has received accelerated FDA approval in HER2-mutated NSCLC. ADCs offer a possible solution to finding a new treatment that could bypass the intracellular resistance mechanism. In this review article, we summarize the mechanism of ADCs and investigational ADCs for EGFR-mutated NSCLC, which include targets to MET amplification, HER3, Trop2, and EGFR, along with other ADC targets being investigated in NSCLC, and discuss future directions that may arise with ADCs in EGFR-mutated NSCLC.

EGFR-mutated NSCLC with progression of disease treatment options on osimertinib is limited, as oligoprogression or CNS metastasis is often treated with radiation therapy while continuing osimertinib (14,15).Those with visceral metastasis beyond oligoprogression or SCLC transformation are treated with chemotherapy (14,15).Immune checkpoint inhibitors (ICIs) have little benefit and significant toxicity in EGFR-mutated NSCLC (16,17).The IMMUNOTARGET registry showed that ICI by itself in EGFR-mutated NSCLC had a shorter median progression-free survival (PFS) of 2.1 months and a lower overall response rate (ORR) of 12% compared to other targetable mutations (18).Attempts to combine EGFR TKIs with ICIs were not successful, as the TATTON study, which involved an EGFR TKI and durvalumab, was discontinued due to toxicity primarily due to interstitial lung disease (ILD) (19).Other combination ICI and EGFR TKI studies (KEYNOTE-021 and Checkmate 012) showed no OS benefit with severe grade 3+ hepatotoxicity in five of seven patients receiving gefitinib in KEYNOTE-021 (19,20).Furthermore, the use of ICI followed by osimertinib led to severe immune-related adverse events in 15% of patients, particularly within 3 months of osimertinib use (17).KEYNOTE-789, a phase 3 randomized controlled trial of chemotherapy and pembrolizumab in TKI-resistant EGFR-mutated patients, showed no PFS or OS benefit (21).The lack of success of ICIs in EGFR-mutated NSCLC has limited treatment options upon progression of first-line EGFR TKI.
Antibody-drug conjugates (ADCs) consist of a monoclonal antibody attached to a cytotoxic drug (22,23).These drugs have highly specific targeting abilities and can effectively kill the targeted cells and theoretically avoid toxicity toward other nontargeted cells (22,23).Gemtuzumab ozogamicin was the first ADC given approval for treatment of patients >60 years with CD33 + acute myeloid leukemia who are not candidates for aggressive therapy (24).In solid tumors, one of the first ADC targets was HER2, a transmembrane protein in the erb-b2 receptor tyrosine kinase 2 (ERBB2).HER2 protein overexpression is observed in about 20% of NSCLC patients (25).Trastuzumab deruxtecan (T-DXd), an ADCtargeting HER2, was the first ADC in NSCLC given FDAaccelerated approval in 2022 (8,26).
The development of T-DXd has led to more antigens being targeted in NSCLC for ADCs.ADCs could circumvent the EGFR cell signaling pathway and provide new treatment options for EGFR-mutated patients who progress on osimertinib.We will discuss ADC structure, mechanism of action, and targets currently in development in patients with EGFR-mutated NSCLC.

ADC structure and mechanism of action
ADC consists of a monoclonal antibody, typically immunoglobulin G, bound to the target antigen of cells in the tumor.The ADC then fuses with the lysosome, leading to cytotoxic "payload" release that causes death of the targeted cancer cell (22,23).When choosing an antigen, antigens chosen to be targets for designed ADCs are antigens that have much greater expression in cancer cells than in noncancer cells.For example, ERBB2, one of the early targets for ADC development, is expressed 100-fold in tumor tissues more than in nontumor tissues (22,23,27).The antibody and payload are connected by the linker and help with ADC stability.Cleavable linkers break down and release the payload based on either a specific pH or a specific lysosomal protease, while noncleavable linkers release the payload following internalization of ADCs by lysosome or proteases (22,23,28).The payload is where the ADC acts on the cancer cell and induces cell death.The payloads include agents that affect microtubule stability, inhibit topoisomerase 1, or cleave the DNA (22,23,29).There is a bystander effect where the drug, after internalization and degradation, is released across the cell membrane to kill adjacent cells (22,23,30).Finally, the drug-to-antibody ratio (DAR) is an important property of ADCs.DAR is the average number of drug molecules (payload) conjugated to the antibody.To calculate DAR, newer ADCs utilize high-performance liquid chromatographyultraviolet spectroscopy, which adjusts for changes to the antibody or heavy/light chain hydrophobicity due to conjugated payloads, though other approaches include liquid chromatographyquadruple time-of-flight mass spectrometry and UV-Vis (31).Earlier ADCs developed had a DAR of 2-4, but newer ADCs such as trastuzumab deruxtecan have a DAR of 8 (31).
AZD9592 is a bispecific ADC targeting EGFR and cMET with a monovalent bispecific IgG platform with an increased affinity toward c-MET to reduce EGFR-related toxicities, along with a topoisomerase I payload (38).AZD9592 monotherapy in vivo in patient-derived xenograft models showed a 30% reduction of tumor in 41% of EGFR mutant NSCLC tumors at 2 mg/kg and a 73% response rate in those treated at 8 mg/kg (38,39).Currently, there is a phase 1 trial on AZD9592 monotherapy in combination with osimertinib in EGFR mutant NSCLC (NCT05647122) (40) (Table 1).

HER3
There remains a large percentage of EGFR-mutated NSCLC patients receiving EGFR TKI with no known identifiable resistance mechanisms (12,13).ERBB3 (HER3) is found in 83% of primary NSCLC tumors and is also expressed in other solid tumors (41).HER3 is found in a greater proportion of EGFR mutant NSCLC compared to EGFR wild-type NSCLC (42).Patritumab deruxtecan (HER3-DXd) is a HER3-directed ADC composed of a monoclonal antibody to HER3 covalently linked to a topoisomerase I inhibitor payload with a DAR ratio of 8 and is cell membrane-permeable, thus allowing cell death both at the target and surrounding tumor cells (43) (Figure 1).In a phase 1 study looking at HER3-DXd in metastatic EGFR-mutated NSCLC with prior EGFR TKI therapy, 57 patients received the dose-recommended HER3-DXd of 5.6 mg/kg q3 weeks (44).The ORR was 39% (95% CI, 26.0-62.4)with a median PFS of 8.2 (95% CI, 4.4-8.3)(44).The ORR in 23 of the 57 patients with known EGFR-related resistance mechanisms, excluding T790M, was 35%.HER3-DXd was well tolerated with a low discontinuation rate (9%, 7/81) (44).Treatment-related ILDs in this study occurred in 5% of patients and resolved in all patients after drug discontinuation (44) (Table 1).
Izalontamab (SI-B001) is another first-in-class novel EGFR × HER3 bispecific antibody.A phase II study looking at SI-B001 plus

EGFR
There have been other ADCs directly targeting EGFR, with mixed results.MRG003 is an anti-EGFR humanized immunoglobulin G1 monoclonal antibody conjugated with monomethyl auristatin E via a valine-citrulline linker (Figure 1).In a phase 1 study of primarily head and neck squamous cell carcinoma, nasopharyngeal carcinoma, and colorectal cancer patients, the ORR was 40%, 44%, and 0%, respectively (NCT04868344) (49).Currently, a phase II study looking at the efficacy and safety of MRG003 is ongoing in EGFR-positive advanced non-small cell lung cancer (NCT04838548) (Table 1).
Depatuxizumab mafodotin (ABT-414), which consists of an EGFR-specifi c humanized anti body, a non-cleavable malemidocaproyl linker, and monomethyl auristatin F, was tested for patients with EGFR-amplified newly diagnosed glioblastoma but discontinued due to a lack of survival benefit (50).AVID-100, which is another EGFR-targeted ADC failed in phase I/II trial in solid tumors due to a lack of efficacy (NCT03094169) (51).M1231 is another ADC that consists of a bispecific antibody targeting Mucin 1 (MUC1) and EGFR with a hemiasterlin-related payload; a current phase I trial in solid tumors is ongoing (NCT04695847) (52) (Figure 1).
AFM24 is a tetravalent bispecific innate cell engager targeting EGFR on tumor cells and CD16A on natural killer cells to enhance antitumor antibody-dependent cellular cytotoxicity (53).A phase 1/ 2 monotherapy evaluating AFM24 in 14 heavily treated EGFR mutant NSCLC patients showed a DCR of 50% and a median duration of therapy of 6.7 weeks (1.0-26.1).There was acceptable s a f e t y w it h o n e i n c i d e n c e o f g r ad e 5 p n e u m o n i t is (NCT04259450) (54).

Additional ADC targets in NSCLC
In advanced NSCLC targeting HER2, ado-trastuzumab emtansine (TDM-1) had an ORR of 44%.However, the median duration of response (DOR) was 3.5 months with a median PFS of 2.8 months and a median OS of 8.1, and thus no further phase III NSCLC trials were pursued due to the limited efficacy (60, 61).T-DXd had an ORR of 55% in advanced HER2-mutant NSCLC with a median PFS of 8.2 months (95% CI, 6.0-11.9months) and a median OS of 17.8 months (95% CI, 13.8-22.1 months).Drug-related ILD did occur in 26% of patients (8).Consequently, a phase II trial comparing T-DXd 5.4 mg/kg versus 6.4 mg/kg dosing showed similar efficacy in both doses, with a decrease in the adjudicated drug-related ILD rate in the 5.4-mg/kg dose (NCT04644237) (62).In regard to other ADC targets, carcinoembryonic antigen-related cell adhesion molecule 5 (CEACAM5) is highly expressed in about 25% of lung cancers, and a novel ADC targeting this is tusamitamab ravtansine, comprising a humanized monoclonal antibody and a cytotoxic maytansinoid, DM4.Phase 1 data of tusamitamab ravtansine showed ORR 20.3% (95% CI, 12.3-31.7)in high expression of CEACAM5 with grade 3 or greater TEAE in 48% of patients (63).Finally, B7-H3 protein is seen in 80% of NSCLC cases with high protein levels associated with poor prognosis and could be a considered a target for ADC development (64).

Future directions
While ADCs provide promise, there are multiple considerations.The first consideration is resistance patterns and treatment upon progression from an ADC.Some mechanisms of ADC resistance include decreased antigen expression after exposure to ADC, as seen in some subjects who had received TDM-1; processing of ADC that may lead to less uptake in the cancer cell; and resistance to the payload (65,66).As ADCs have an increased risk for ILD toxicity, there may be a risk for pneumonitis if patients were to go back to an EGFR TKI afterward (67).This is important as newer generation of TKIs are being developed, such as BLU-945 to address EGFR C797X resistance, and questions arise as to sequencing between drugs (68,69).
Another consideration is weighing drug efficacy versus drug toxicity.The incidence of ILD in the key trials has been about 20%, and there are other notable toxicities that can significantly alter the quality of life, such as 20% ocular toxicity seen in TROPION-Lung02 (44,70).Subsequent analysis of patient-reported outcomes will be vital in determining the benefit of drug efficacy versus drug toxicity.
CNS response rate is an important consideration.EGFRmutated NSCLC leads to brain metastasis in about 50%-60% of patients, so finding an agent with CNS activity is vital.Earlier ADCs did not have good blood-brain barrier penetration due to a lack of homogeneity of the ADC along with a suboptimal drug-to-antibody ratio (71).The phase 1 HER3-DXd study in NSCLC patients has an ORR of 36.4% in patients with brain metastasis and a median DOR of 7.3 months, suggesting some activity, but more work needs to be done, particularly given the high risk of CNS metastasis in these patients (72).
In addition, up to 15% of EGFR-mutated patients have the risk of small-cell lung cancer transformation on progression (12, 13).There have not been any reported trials on ADCs in these patients, but deltalike ligand 3 (DLL-3) is a target expressed in 80% of SCLCs.Rovalpituzumab tesirine was an ADC in development in SCLC that was discontinued due to lack of benefit, but bispecific T-cell engagers such as tarlatamab have shown promising responses in heavily treated SCLC (73,74).However, the mechanism of SCLC and squamous cell transformation in these patients needs to be understood so that we can focus on a specific target for this population.
Finally, combinations with ADCs are a consideration.Many trials combine chemotherapy, but with EGFR-mutated NSCLC, it will be interesting to see if a combination of ADC with EGFR TKI as studied in HER3-DXd with osimertinib will prove to be more efficacious and not more toxic than ADC monotherapy, and if so, if this combination will be more effective than ADC with chemotherapy (NCT04676477).

Conclusion
There has been tremendous progress in EGFR-mutated NSCLC since the approval of erlotinib in 2013, but with that come challenges, notably acquired resistance and treatment upon disease progression (10).ADCs are an increasingly studied drug class with more antigen targets being found.With trastuzumab deruxtecan approved for HER2-mutated NSCLC, there are promising developments in other ADCs targeting HER3, MET amplification, and EGFR directly.Future challenges involve the combination of ADCs with other drugs, sequencing of drugs, toxicity, particularly ILD incidence, and controlling brain metastasis in patients with CNS involvement.

TABLE 1
Current/key trials of ADCs in EGFR-mutated NSCLC.