Traditionally, the foundation of cancer treatment has been cytotoxic chemotherapy, surgery, and radiation therapy. However, in recent years, with the advent of immunotherapy and the ability to target specific molecular agents, the treatment of cancer has been revolutionized and is no longer limited only to the conventional chemotherapeutic agents [1]. In 1975, hybridoma technology helped to greatly advance the precision tumor targeting via monoclonal antibodies (mAbs). The concept of targeted delivery of drugs was first formulated in the early 1900s by Paul Ehrlich when he envisioned the concept of a magic bullet which would ensure that the drugs achieve their intended cellular targets while remaining inoffensive to the normal tissues [2]. Hereafter, an innovative conception of antibody–drug conjugate was proposed that conglomerates both the target specificity of mAbs and the anti-tumor properties of cytotoxic molecules, thereby improving the therapeutic window [3]. Nonetheless, it was only in the 1980s that the first ever clinical trials of ADCs were designed that unfortunately did not elicit satisfying results due to minimal efficacy and high toxicity [4]. Consequently, in the year 2000, a substantial success was achieved in the development of ADC with the advent of the anti-CD33-targeted agent gemtuzumab ozogamicin for adults with acute myeloid leukemia [5]. A decade later, the next ADC, brentuximab vedotin, was approved for the treatment of classical Hodgkin lymphoma and systemic anaplastic large cell lymphoma [6]. In 2013, the first approved ADC to treat solid tumors was the human epidermal growth factor receptor 2 (HER2)-targeted ADC, ado-trastuzumab emtansine (T- DM1), to treat metastatic breast cancer [7].

Currently, there are eleven FDA-approved antibody–drug conjugates available in the market. Furthermore, over 100 ADCs are in the diverse stages of development in clinical trials at the present time. Notably, a few novel ADCs have established substantial anti-tumor activity in many tumor types that share the expression of the targeted antigen, thus establishing a histology-agnostic activity of these compounds [8]. For instance, HER2 overexpressing breast, gastric and colorectal cancers, and Her-2 mutated lung cancers have demonstrated a significant response to the anti-Her2-targeting ADC, trastuzumab deruxtecan [9].

2. Antibody–Drug Conjugates

2.1. Mechanism of Action

Antibody–drug conjugates are comprised of an antibody, a cytotoxic payload, and a chemical linker [10]. Once the ADC reaches the target cells, the mAb component recognizes and binds to the cell surface antigens, and the ADC–antigen complex is then internalized within the cancer cell by endocytosis to form an early endosome, which, following a maturation, forms late endosomes and finally fuses with lysosomes. The cytotoxic drug payload is then released from the mAb via either a chemical reaction or enzyme digestion in the lysosomes, and exerts its cytotoxic effect, causing cell apoptosis or death [11]. In addition to the cytotoxic properties from the payload, the Fc portion of the monoclonal antibody aids in immune-related cytotoxicities, such as antibody-dependent cell mediated cytotoxicity (ADCC), antibody-dependent phagocytosis (ADP), and complement-dependent cytotoxicity (CDC) [12]. Genetic engineering technologies have advanced to enhance the effector function of the antibody in the Fc region [12]. Additionally, the binding of the antibody component of ADC with the specific antigen epitope of cancer cells can inhibit the downstream signal transduction of the antigen receptor [13]. For example, in T-DM1, the antibody trastuzumab binds to the HER2 receptor of cancer cells and inhibits the development of heterodimer between HER2 and HER1, HER3 or HER4, which blocks the signal transduction pathways of cell survival and proliferation to induce the cell apoptosis [14].

2.2. Key Components

Each component, as well as the conjugation methods of an ADC, can affect the final efficacy and safety of ADC.

2.3. Target Antigen

An appropriate selection of target antigen is imperative to the design of an ADC. First, the antigen should be expressed, either exclusively or predominantly, in the tumor cells to reduce the off-target toxicity [15]. Secondly, the binding to the target antigen should ideally lead to the internalization of the antigen–antibody complex. Additionally, it should ideally be on the surface rather than intracellular for it to be recognized, and lastly, it should not be secretory, since a secreted antigen in the circulation would cause the undesirable ADC to bind outside of the tumor sites ^[16][17][16,17].

2.4. Antibody

An ideal antibody moiety should also facilitate an effective internalization, have high antigen affinity, preserve long plasma half-life, and demonstrate low immunogenicity [18]. The mAb are large-sized and account for over 90% of the mass of any given ADC. This is favorable because it encounters reduced distribution or permeation into healthy tissue, including those normally functioning as metabolizing and eliminating organs [19]. However, no such problem is encountered at the tumor site as the vasculature in the tumor is characteristically leaky and allows the distribution and permeation of the ADCs to the tumor cells [20].

2.5. Linker

Diverse ADC properties are also impacted by linker chemistry, including specificity, stability, potency, and toxicity [21]. Depending on the methods of releasing the payload in cells, there are two types of linkers, including cleavable and non-cleavable [22]. The cleavable linkers are either chemically labile (hydrazone bond and disulfide bond) or enzymatically labile. Hydrazone linkers are generally stable in alkaline environments and are hydrolyzed in low pH environments, such as that in the lysosome and endosome. Hence, the cleavage of ADCs with hydrazone linkers occurs predominantly in the lysosome and endosome upon internalization, with occasional hydrolysis in the plasma, resulting in off-target, systemic toxicity [23]. Similarly, a disulfide bond linker can be stable in the plasma while specifically releasing the active payloads in the cancer cells with an elevated reductive glutathione level [24]. The enzyme sensitive linkers are sensitive to the lysosomal protease that is generally overexpressed in cancer cells, enabling an accurate drug release in the cells after internalization [25]. ADCs with non-cleavable linkers are resistant to chemical or enzymatic digestion in the plasma and will require complete degradation of the antibody within the late endosomes and lysosome to release the payload. Therefore, ADCs with non-cleavable linkers may have the lowest off-target systemic toxicity due to increased plasma stability ^[26][27][26,27], and thus they are most suitable in the treatment of tumors with homogenous antigen expression. Some of the ADCs have been engineered to have desirable “off-target effect” for “by-stander killing”, extending the cytotoxic effect to the low or negative antigen-expressing cells in the tumor proximity. For this mechanism to work, several characteristics of the ADC molecules are crucial: namely, a cleavable linker and a non-polar, freely membrane-permeable payload [28]. For instance, Trastuzumab deruxtecan (T-DXd) has a cleavable enzymatic linker as opposed to trastuzumab emtansine that has a thioether non-cleavable linker. Payloads such as deruxtecan, MMAE, or maytansinoid DM4 are cell membrane-permeable, and can diffuse out of the cells after cleavage in the lysosomes. Hence, by virtue of its component properties, TDXd has bystander effect, thus is effective even in low-Her2-harboring tumors, whereas trastuzumab emtansine due to its properties has a lower toxicity profile. Conversely, to reduce the undesirable systemic toxicity from payload molecules permeating out of the tumor cells, ionizable payloads (e.g., containing carboxylic acids) can be used [29]. As of now, trastuzumab emtansine is the only approved ADC that uses a non-cleavable linker, while the rest of ADCs are equipped with cleavable linkers. Modifications and new developments of linker bonds and structures are evolving constantly.

2.6. Cytotoxic Drugs

The cytotoxic payloads should ideally have the following properties: High potency, in vitro high cytotoxic activity (sub nanomolar half maximal inhibitory concentration (IC50) value), high stability in the systemic circulation, sufficient solubility in the aqueous environment of antibody and biochemical properties to allow easier conjugation to the antibody, low immunogenicity, small molecular weight, and a long half-life ^[30][31][30,31]. In the current approved ADCs, there are mainly two classes of cytotoxic drugs used as payloads: the microtubule inhibitors or the DNA damaging agents. Auristatins and maytansines payloads are both cytotoxic agents that work as tubulin inhibitors. Auristatin is a dolastatin synthetic analog. There are two auristatin derivatives: one is monomethyl auristatin E (MMAE) and the other is monomethyl auristatin F (MMAF). These two products differ structurally wherein the phenylalanine present at the C-terminus renders MMAF membrane-impermeable, whereas the MMAE can exit the cell and thus diffuse to nearby cells and kill them through the bystander effects [32]. The ADCs with MMAE payload are Brentuximab Vedotin, Polatuzumab Vedotin, Enfortumab Vedotin, and Tisotumab Vedotin, while Belantamab mafodotin uses MMAF as a cytotoxic payload (Belantamab is now withdrawn from the market). Maytansinoids are natural cytotoxic agents isolated from the cortex of Maytenus serrata, which possesses a macrolide structure. Maytansionoid payloads, DM1 and DM4, are a component of Trastuzumab Emtansine and Mirvetuximab Soravtansin, respectively ^[32][33][32,33]. Calicheamicins, Pyrrolobenzodiazepines and topoisomerase inhibitors are DNA-damaging agents that act through DNA double strand breaks, crosslinking, and intercalation, respectively [34]. Both Gemtuzumab ozogamicin and Inotuzumab Ozogamicin have N-acetyl gamma calicheamicin as a payload. Calicheamicins belong to a class of potent anti-tumor antibiotics that cleave the DNA in a site-specific, double-stranded manner. Pyrrolobenzodiazepines are another class of antibiotics derived from Streptomyces species and is used as a cytotoxic payload in Loncastuximab Tesirine [34]. SN-38 and Deruxtecan are topoisomerase inhibitors that are the cytotoxic components of Sacituzumab Govitecan and Trastuzumab Deruxtecan, respectively [35].

2.7. Conjugation

In addition to the choice of the antibody, the linker, and the payload, the method of conjugation is also important for the successful structure of ADCs. The lysine and cysteine residues on the antibody provide the accessible reaction sites for conjugation ^[36][37][36,37]. A varying number (0–8) of small-molecule toxins may be attached to an antibody, as the conventional conjugation methods are random, resulting in a wide drug–antibody ratio (DAR) distribution [38]. The ideal DAR is 2–4. A low DAR can lower the efficacy, while a high DAR may increase the drug potency, but it also could negatively affect antibody structure, stability, and antigen binding, leading to faster clearance and decrease in overall clinical activity [39].

3. FDA Approved ADCs

3.1. Gemtuzumab Ozogamicin

Gemtuzumab ozogamicin (GO, also known as CMA-676) is a humanized anti-CD33 monoclonal antibody linked by an acid-labile hydrazone cleavable linker to the cytotoxic agent N-acetyl gamma calicheamicin that belongs to the family of enediyne antibiotics. The monoclonal antibody binds to the CD33 antigen, thereby leading to the internalization and release of the cytotoxic agent, which induces double-strand DNA breaks and cell death [40]. On 17 May 2000, GO received accelerated approval for the treatment of older patients with relapsed CD33-positive acute myeloid leukemia (AML), but the approval later was withdrawn based on safety and efficacy data in 2010 [41]. The original recommended dose was 9 mg/m², a total of two doses administered 14 days apart. Fatal hepatotoxicity and veno-occlusive disease (VOD) in patients who received GO before or after hematopoietic stem cell transplantation (HSCT) was issued as a black box warning [42]. The withdrawal, however, was based on a randomized clinical trial evaluating GO 6 mg/m² in combination with induction chemotherapy in patients ≤ 60 years with newly diagnosed AML. The trial failed to demonstrate a clinical benefit, but instead showed a higher rate of fatal induction toxicities compared with chemotherapy alone [43]. Recent pharmacokinetic studies suggest that the reason for the serious toxicity was that the original dose was too high [41]. Later, on 1 September 2017, the U.S. Food and Drug Administration approved GO (Mylotarg, Pfizer) for the treatment of relapsed or refractory (R/R) CD33-positive AML in adults as a monotherapy, as well as for newly diagnosed CD33-positive AML in adults and pediatric patients 1 month and older in combination with chemotherapy. The recommended dose is 3 mg/m² [44]. For R/R CD33-positive AML, the single-arm phase II (MyloFrance 1) trial on 57 patients showed GO monotherapy resulted in a CR/CRp rate of 26% (95% confidence interval (CI) 16–40%) and the median relapse-free survival was 11.6 months [45]. For newly diagnosed de novo AML, a multicenter, randomized, open-label phase 3 study (ALFA-0701) of 280 patients aged from 50 to 70 years of age showed a significant event-free survival (EFS) benefit of 17.3 months for patients receiving GO with combination chemotherapy vs. 9.5 months for those receiving chemotherapy alone, with a hazard ratio of 0.56 (95% CI: 0.42, 0.76). There was no significant difference in the rate of complete remission (CR) (70.4% vs. 69.9%), but the median overall survival (OS) was 27.5 months [95% CI: 21.4–45.6] in the GO arm and 21.8 months (95%CI: 15.5–27.4) in the control arm [Hazard Ratio (HR), 0.81; 95%CI: 0.60–1.09; 2-sided p = 0.16) [46]. Furthermore, a multicenter, randomized study of 1022 patients (511 each arm) with newly diagnosed AML in patients of ages from 0 to 29 years (AAML0531 (NCT00372593)) showed that GO, in combination with chemotherapy, significantly improved 3-year EFS (53.1% vs. 46.9%; HR 0.83; 95% CI: 0.70 to 0.99; p = 0.04), but not 3-year OS (69.4% vs. 65.4%; HR 0.91; 95% CI: 0.74 to 1.13; p = 0.39). There was no difference in CR (88% vs. 85%; p = 0.15); however, a 3-year relapse risk (RR) was significantly less among GO (32.8% vs. 41.3%; HR, 0.73; 95% CI: 0.58 to 0.91; p = 0.006) [47]. Based on the EFS benefit, the FDA later extended the indication of GO for newly diagnosed CD33-positive acute myeloid leukemia (AML) to include pediatric patients 1 month and older, on 16 June 2020 [48].

3.2. Brentuximab Vedotin

Brentuximab vedotin (BV) is a CD30-directed ADC consisting of the chimeric IgG1 antibody cAC10, specific for human CD30 and a microtubule-disrupting agent, microtubule-binding auristatin, monomethyl auristatin E (MMAE), with a protease-cleavable linker [49]. The proteolytic cleavage after internalization releases MMAE, which induces cell cycle arrest and apoptosis by disrupting the microtubular network [50]. Due to the advantage of the bystander effect from the membrane permeability of MMAE, brentuximab vedotin is also effective in the histologically heterogenous tumor expression of CD30 [45]. On 19 August 2011, the U.S. Food and Drug Administration (FDA) provided accelerated approval of BV (Adcetris, Seattle Genetics) for two indications [51]. The first approved use of Brentuximab vedotin was in patients with Hodgkin lymphoma, who either relapsed after two or more prior lines of therapy, after autologous stem cell transplant (ASCT), or those who were not ASCT candidates. A phase II trial that involved 102 patients with relapsed and refractory Hodgkin lymphoma and treated with single-agent BV suggested an overall response rate (ORR) of 75% (95% CI, 64.9% to 82.6%), with CR in 34% of patients (95% CI, 25.2% to 44.4%). The median progression-free survival (PFS) was 5.6 months (95% CI, 5.0 to 9.0 months), the median duration of response in patients achieving a CR was 20.5 months, and the median OS was 22.4 months [52]. Second, it was approved for the treatment of patients with systemic anaplastic large-cell lymphoma (ALCL) after failure of at least one prior multi-agent chemotherapy regimen. In a phase II study of 58 patients with relapsed or refractory systemic anaplastic large-cell lymphoma, who had been treated with single-agent BV, ORR was 86% (95% CI, 74.6% to 93.9%), with 57% CR. The median duration of overall response was 12.6 months, the median PFS was 13.3 months (95% CI, 6.9 months to NE), and the OS was not reached [53]. The approval was then expanded for classical Hodgkin lymphoma at a high risk of relapse after or progression after consolidation with ASCT [54]. AETHERA was a phase 3 clinical trial with single-agent BV administered every 3 weeks following ASCT, compared to a placebo for up to 16 cycles, that showed statistically significant improved PFS in patients receiving BV 42.9 months vs. 24.1 months, respectively (hazard ratio [HR] 0.57, 95% CI 0.40–0.81; p = 0.0013) [55]. The FDA granted regular approval of BV on 9 November 2017 for the treatment of CD30-expressing mycosis fungoides (MF) or primary cutaneous anaplastic large cell lymphoma (pcALCL), for patients who have received prior systemic therapy [56]. This was based on a phase 3, randomized, open-label, multicenter clinical trial called ALCANZA, in which 131 patients with MF or pcALCL who, after previously receiving one prior systemic therapy, then received BV. The objective response was achieved in 56.3% of patients (36/64 patients) with BV vs. 12.5% (8/64) with the physician’s choice seen after a median follow-up of 22.9 months (95% CI 29·1–58·4; p < 0.0001) [57]. Later, on 20 March 2018, BV in combination with chemotherapy was approved for use for the treatment of adult patients with newly diagnosed Stage III/IV classical Hodgkin lymphoma [58]. This approval was based on a trial called ECHELON-1, that included 1344 patients that were randomized 1:1 to receive either BV + AVD (doxorubicin, vinblastine, and dacarbazine) or ABVD (doxorubicin, bleomycin, vinblastine, and dacarbazine). After a median follow-up of 24.6 months, 2-year PFS in the BV + AVD vs. ABVD was 82.1% and 77.2%, respectively (HR 0.77; 95% CI, 0.60 to 0.98; p = 0.04), and there were 28 vs. 39 deaths with BV + AVD and ABVD, respectively (HR for interim overall survival is 0.73 [95% CI, 0.45 to 1.18]; p = 0.20) [59]. The FDA, on 16 November 2018, granted approval to BV with chemotherapy for newly diagnosed systemic anaplastic large cell lymphoma and other CD30-expressing peripheral T-cell lymphomas (PTCL) [60]. This was based on ECHELON-2 that randomized 452 patients to BV + CHP (cyclophosphamide, doxorubicin, and prednisone) and CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) in a 1:1 ratio. The median PFS in the BV + CHP group was 48.2 months and in the CHOP group was 20.8 months (hazard ratio 0.71 [95% CI 0.54–0.93], p = 0.0110) [61]. On 10 November 2022, the FDA approved BV with doxorubicin, vincristine, etoposide, prednisone, and cyclophosphamide for previously untreated high-risk classical Hodgkin lymphoma (cHL) in pediatric patients more than 2 years of age. This was based on a trial of 600 patients randomized in a 1:1 ratio to brentuximab vedotin plus doxorubicin, vincristine, etoposide, prednisone, and cyclophosphamide [brentuximab vedotin + AVEPC] vs. A + bleomycin +V + E + P + C [ABVE-PC]. The 3-year EFS was 92.1% in the brentuximab vedotin group vs. 82.5% in the ABVE-PC group after a median follow-up of 42.1 months (HR 0.41; 95% CI, 0.25 to 0.67; p < 0.001) [62].

3.3. Ado-Trastuzumab Emtansine

Ado-trastuzumab emtansine is a HER2-targeted antibody–drug conjugate. The antibody component is the humanized anti-HER2 IgG1, and trastuzumab, and the small molecule cytotoxin is DM1. The linker is non-cleavable and hence stable in both the circulation and the tumor microenvironment; thus ado-trastuzumab emtansine, upon binding to the sub-domain IV of the HER2 receptor, undergoes lysosomal proteolytic degradation, resulting in the release of DM-1. The binding of DM-1 to tubulin then disrupts microtubule networks in the cell and hence leads to cell cycle arrest and apoptotic cell death [63]. The FDA, on 22 February 2013, approved Trastuzumab emtansine (Kadcyla, Roche) for patients with metastatic HER2-positive breast cancer, who had previously received trastuzumab and a taxane. [64]. This was based on the phase III EMILIA trial. In this trial, 991 patients with HER2-positive metastatic breast cancer were randomized to receive ado-trastuzumab emtansine vs. lapatinib with capecitabine. The study demonstrated an ORR of 43.6% vs. 30.8% (p < 0.001), a median PFS of 9.6 months vs. 6.4 months (HR 0.65; 95% confidence interval [CI], 0.55 to 0.77; p < 0.001), and the median overall survival of 30.9 months vs. 25.1 months; (HR 0.68; 95% CI, 0.55 to 0.85; p < 0.001) [65]. Later, on 3 May 2019, the FDA approved TDM1 as a single agent for adjuvant treatment of patients with HER2-positive breast cancers that have residual disease after receiving neoadjuvant trastuzumab-based therapy. This was based on a randomized open-label phase III trial KATHERINE [66]. This trial enrolled a total of 1486 patients that were randomized to either receive TDM1 or trastuzumab as an adjuvant treatment for HER2-positive early breast cancer that have residual disease. This study demonstrated a 3-year invasive disease-free survival (IDFS) benefit in patients who received ado-trastuzumab emtansine, compared with those who received trastuzumab (88.3% vs. 77%) (HR 0.50; 95% confidence interval, 0.39 to 0.64; p < 0.001) [67].

3.4. Inotuzumab Ozogamicin

Inotuzumab ozogamicin is comprised of an anti-CD22 humanized monoclonal antibody that is linked to calicheamicin, a cytotoxic antibiotic by an acid-labile hydrazone linker [68]. CD22 is an endocytic receptor that is a specific marker of B-cell acute lymphocytic leukemia (ALL) and is expressed in over 90% of patients with B-cell malignancies [69]. Once the antibody–drug conjugate binds to the CD22 receptor, the complex becomes internalized inside the targeted cell, triggering the release of calicheamicin, which induces cellular apoptosis by binding with the minor groove of double-helical DNA and causing site-specific double-stranded DNA cleavage ^[68][70][68,70]. Inotuzumab ozogamicin (Besponsa, Pfizer) was given FDA approval for the treatment of adults with relapsed or refractory B-cell precursor ALL on 17 August 2017 [71]. A phase 3 INTO-VATE ALL trial randomized a total of 326 patients to receive either inotuzumab ozogamicin or standard intensive chemotherapy. The CR rate was 80.7% (95% CI: 72.1, 87.7) vs. 29.4% (95% CI: 21.0, 38.8), the median PFS was 5.0 months [95% CI: 3.7, 5.6] vs. 1.8 months [95% CI: 1.5, 2.2] (hazard ratio, 0.45 [97.5% CI, 0.34 to 0.61]; p < 0.001), and the median overall survival was 7.7 months (95% CI: 6.0, 9.2) vs. 6.7 months (95% CI: 4.9 to 8.3) (hazard ratio, 0.45 [97.5% CI, 0.34 to 0.61]; p < 0.001) in inotuzumab ozogamicin vs. standard chemotherapy, respectively ^[68][72][68,72].

3.5. Polatuzumab Vedotin Piiq

Polatuzumab vedotin is a conjugate composed of an anti-CD79b monoclonal antibody linked via a protease-cleavable linker to monomethyl auristatin E (MMAE), which is a potent microtubule inhibitor, used similarly in the ADC of Brentuximab vedotin ^[72][73][72,73]. Polatuzumab vedotin, (Polivy, Roche) in combination with bendamustine and rituximab, was granted accelerated approval on 10 June 2019 for the treatment of relapsed or refractory diffuse large B-cell lymphoma after receiving two or more therapies [74]. This approval was based on an open-labeled phase 2 trial that included 80 patients with relapsed or refractory DLBCL after at least one prior regimen, who were randomized 1:1 to either P + BR vs. BR alone. This trial showed an ORR of 45% vs. 17.5%, a CR of 40% (95% CI: 25–57%) vs. 17.5% (p = 0.026), a median PFS of 9.5 vs. 3.7 months (HR, 0.36; 95% CI, 0.21 to 0.63; p < 0.001), and a median OS of 12.4 months vs. 4.7 months (HR, 0.42; 95% CI, 0.24 to 0.75; p < 0.002) with P + BR vs. BR alone after a median follow-up of 22.3 months [75].

3.6. Enfortumab Vedotin

Enfortumab vedotin (EV) is a nectin-4 targeted antibody–drug conjugate that is comprised of a human monoclonal antibody specific for nectin-4 and linked by a protease cleavable linker with monomethyl auristatin E (MMAE) [76]. Nectin-4 is a cell-adhesion molecule that is overexpressed in 97% of urothelial carcinomas and is associated with tumor growth and proliferation [77]. Directed delivery of the cytotoxic payload, monomethyl auristatin E, results in cell cycle arrest and apoptosis [78]. Enfortumab vedotin (Padcev, Seagen) was granted accelerated approval by the FDA on 18 December 2019 for use in adult patients with locally advanced or metastatic urothelial carcinoma who had previously received treatment with either a programmed death receptor-1 (PD-1) inhibitor or a programmed death ligand receptor-1 (PD-L1) inhibitor, along with a platinum-containing chemotherapy agent [79]. The agent was approved after a phase II trial consisting of 125 patients with locally advanced or metastatic urothelial cancer who had previously received treatment with either a PD-1 or PD-L1 inhibitor and a platinum-containing chemotherapy agent. The trial found that the objective response rate (ORR) was 44% (95% CI: 35.1, 53.2), with 12% having a complete response and 32% having a partial response, with a medium response duration of 7.6 months [80]. On 9 July 2021, the FDA granted regular approval for enfortumab vedotin to be used in adults with advanced or metastatic urothelial carcinoma who had been treated with either a PD-1 inhibitor or PD-L1 inhibitor and platinum-containing therapy, or those who were ineligible for cisplatin-containing therapy and had been treated with at least one prior line of treatment [81]. The regular approval was granted based on trial EV-301, which was done to corroborate the benefit of the aforementioned 2019 approval. EV-301 was a randomized phase III study and randomized 608 participants with advanced or metastatic urothelial cancer who received PD-1 or PD-L1 inhibitor and platinum-based therapy to single-agent Enfortumab vedotin or a single chemotherapy agent of the investigator’s choice (docetaxel, paclitaxel, or vinflunine). The trial revealed that enfortumab vedotin significantly lengthened survival and showed superior efficacy when compared to traditional single-agent chemotherapy, with the median overall survival and progressive-free survival in the enfortumab group being 12.9 months (95% CI: 10.6, 15.2) and 5.6 months (95% CI: 5.3, 5.8), compared with 9.0 months (95% CI:8.1, 10.7) and 3.7 months (95% CI:3.5, 3.9) in those receiving traditional chemotherapy (95% CI: 8.1, 10.7), respectfully. ^[65][70][65,70]. Additionally, the efficacy of enfortumab in adults with urothelial cancer who received PD-1 or PD-L1 inhibitor therapy, but were ineligible for platinum-containing therapy, was assessed in cohort 2 of EV-201 (n = 89), with the overall response rate determined to be 51% (95% CI: 39.8, 61.3) and with a median response duration of 13.8 months [76]. The FDA then granted accelerated approval to enfortumab vedotin plus pembrolizumab on 3 April 2023 for patients with locally advanced or metastatic urothelial carcinoma and who were not eligible for cisplatin-containing chemotherapy [82]. The approval was based on the cohort A/combined dose-escalation and cohort K of the phase 1b/2 EV-103/Keynote-869 (NCT03288545) trial. In the 121 treated patients, after a median follow-up of 44.7 months and 14.8 months in cohort A/combined dose-escalation and cohort K, respectively, the objective response rate was 68% (95% CI, 59–76%) with a complete response rate of 12%. The median DOR in the dose-escalation cohort plus cohort A was 22.1 months (range, 1.0+ to 46.3+) and was not yet reached (range, 1.2 to 24.1+) for cohort K ^[83][84][83,84]. There is currently an ongoing phase 3 trial EV-302 trial (NCT04223856) to confirm this approval.

3.7. Trastuzumab Deruxtecan

Fam-trastuzumab deruxtecan (T-DXd) is an antibody–drug conjugate composed of an anti-HER2 antibody trastuzumab, a cleavable tetrapeptide-based linker, and a cytotoxic topoisomerase I inhibitor (an exatecan derivative) [85]. Extensive preclinical studies have predicted T-DXd to be effective in tumor cells with both a high or low Her2 expression status or Her2 mutated tumor cells. There are multiple explanations for the same. Firstly, the ADC has a high drug–to–antibody ratio of 8, and therefore it carries a high potency payload; secondly, the released payload has a high membrane permeability, allowing it to enter neighboring tumor cells, resulting in a bystander effect; and thirdly, it has a high stability in plasma due to the novel tetrapeptide-based linker [86]. In addition, the payload has a short half-life, thereby minimizing the systemic toxicity [80]. Moreover, in vitro and in vivo pharmacologic activities have shown that T-DXd showed potent anti-tumor activity against tumor cells that were resistant to T-DM1, which is likely due to a higher susceptibility of T-DM1 to p-glycoprotein mediated efflux, while T-DXd was not subject to this drug efflux mechanism. Hence, Trastuzumab deruxtecan has shown significant anti-tumor activity in several HER2-expressing cancers, including HER2-positive and HER2-low breast cancer, HER2-positive gastric cancer, and NSCLC with HER-2-activating mutations, regardless of Her2 protein expression and HER2-overexpressing colorectal cancer [87]. Fam-trastuzumab deruxtecan (Enhertu, Daiichi Sankyo) received accelerated FDA approval in December 2019 for patients with unresectable or metastatic HER2-positive breast cancer, who had previously been treated with two or more prior HER2 targeted regimens in the metastatic setting [88]. This was based on the results of DESTINY-Breast01 trial that enrolled 253 patients with HER2-positive, unresectable, or metastatic breast cancer, and who had received previous treatments including trastuzumab emtansine. In 184 patients who received trastuzumab deruxtecan after a median of six previous treatments, the ORR was 60.9% (95% [CI], 53.4 to 68.0), the median PFS was 16.4 months (95% CI, 12.7 to not reached), and the overall survival was 86.2% (95% CI, 79.8 to 90.7) at 12 months; the median overall survival was not reached [89]. On 4 May 2022, the FDA granted regular approval to trastuzumab deruxtecan for patients with unresectable or metastatic HER2-positive breast cancer who had received a prior HER2- targeted regimen in any setting [90]. This was based on DESTINY-Breast03, a phase 3, multicenter, randomized trial that enrolled 524 patients to compare trastuzumab deruxtecan vs. trastuzumab emtansine in patients with HER2-positive metastatic breast cancer, previously treated with trastuzumab and a taxane. The ORR was 79.7% (95% CI, 74.3 to 84.4) vs. 34.2% (95% CI, 28.5 to 40.3) in trastuzumab deruxtecan vs. trastuzumab emtansine. At 12 months, the percentage of those who were alive without disease progression was 75.8% in the trastuzumab deruxtecan group (95% confidence interval [CI], 69.8 to 80.7) vs. 34.1% with trastuzumab emtansine (95% CI, 27.7 to 40.5) (HR for progression or death from any cause, 0.28; 95% CI, 0.22 to 0.37; p < 0.001), and the percentage of patients who were alive at 12 months was 94.1% (95% CI, 90.3 to 96.4) vs. 85.9% (95% CI, 80.9 to 89.7), respectively (HR for death, 0.55; 95% CI, 0.36 to 0.86; p value not reached) [91]. Trastuzumab deruxtecan was approved for the treatment of locally advanced or metastatic HER2-positive gastric or gastroesophageal junction adenocarcinoma in patients who had received a prior trastuzumab-based regimen on 15 January 2021 [92]. This was based on DESTINY-Gastric01, a multicenter, randomized trial of patients with HER2-positive locally advanced, metastatic gastric, or GEJ adenocarcinoma, and who progressed on at least two prior regimens. A total of 125 patients were randomized to receive T-DXd, while 62 received chemotherapy of the physician’s choice. This study demonstrated that the ORR was 51% vs. 14% (p < 0.001), and the median OS was 12.5 and 8.4 months (HR for death, 0.59; 95% confidence interval, 0.39 to 0.88; p = 0.01) in the T-DXd and chemotherapy groups, respectively [93]. On 5 August 2022, the FDA further approved T-DXd for unresectable or metastatic HER2-low as demonstrated by IHC 1+ or IHC 2 + /ISH− breast cancer that had received prior chemotherapy [94]. The DESTINY-Breast04 trial randomized 557 patients with unresectable or metastatic HER2-low breast cancer in a 2:1 ratio to receive T-DXd or the physician’s choice of chemotherapy. This study demonstrated an objective response rate of 52.3% (95% CI, 47.1 to 57.4) vs. 16.3% (95% CI, 11.3 to 22.5); the median PFS was 9.9 months and 5.1 months (HR 0.50; p < 0.001), and the OS was 23.4 months and 16.8 months, (HR 0.64; p = 0.001) in the T-DXd and chemotherapy arms, respectively [95]. The FDA extended and granted accelerated approval for unresectable or metastatic non-small cell lung cancer (NSCLC) with activating HER2 (ERBB2) mutations after progressing on prior therapy on 11 August 2022 [96]. This accelerated approval based on the DESTINY-Lung02 showed an ORR of 58% (95% CI: 43, 71) and a median DOR of 8.7 months (95% CI: 7.1, not estimable [NE]) in patients with HER2-mutant disease [87].