Topoisomerase inhibitors have been widely used as anticancer drugs over the past two decades. Camptothecin and its derivatives have emerged as highly promising anticancer drugs as topoisomerase I inhibitors that selectively capture the Topoisomerase I cleavage complex. To increase the accumulation of Topoisomerase I inhibitors in cancer cells by targeting tumors, topoisomerase inhibitor-based antibody-drug conjugates (ADCs) have been widely developed.
Topoisomerase I inhibitors based ADCs, such as Trastuzumab deruxtecan (Enhertu) and Sacituzumab govitecan (Trodelvy), have shown enhanced therapeutic efficacy compared to typical anti-cancer ADCs like T-DM1.
Topoisomerase I inhibitors
Topoisomerase enzymes are located in the cell nucleus and function to control and repair DNA supercoiling and entanglements occurring during DNA opening, upstream transcription and replication. These catalytic enzymes are able to cleave, repair supercoils and re-ligate DNA strands. Topoisomerase inhibitors are divided into two families based on their cleavage activity: topoisomerases I is able to cleave DNA single strands, while topoisomerases II is able to cleave DNA double strands. TOP inhibitors specifically bind to the interacting surfaces of the DNA-topoisomerase complexes, which blocks the repair function of the topoisomerase, leading to DNA damage and ensuing apoptotic cell death.
Figure 1. Topoisomerase I and Topoisomerase I inhibitor mechanism. 
As a typical topoisomerase I inhibitor, camptothecin (CPT) is a pentacyclic pyrroloquinoline alkaloidplant alkaloid that has been firstly isolated and identified from the chinese tree Camptotheca acuminata (happy tree). The poor solubility of camptothecin in aqueous solutions greatly limits its use in cancer therapy. It was not until the successful approval of irinotecan and topotecan that a strong interest in camptothecin analogues as topoisomerases I inhibitors was stimulated. Since then, a large number of camptothecin derivatives and E-ring-modified analogues have been investigated in the clinic. So far, among the topoisomerase I inhibitors, only camptothecin derivatives have been used as ADC payloads, with two CPT derivatives successfully conjugated to antibodies and approved: DXd, and the active metabolite of irinotecan, SN-38.
Figure 2. Structures of camptothecin (CPT), irinotecan (CPT-11) and SN-38.
SN-38 Based ADC
SN38, the active metabolite of irinotecan (CPT-11), has an anticancer efficacy 100–1000 folds more than CPT-11 in vitro. Irinotecan has been approved by the FDA for the treatment of various solid tumors such as gastrointestinal malignancies, glioblastomas and cervical cancer.
IMMU-132 (Trodelvy) is a Trop-2-directed ADC composed of sacituzumab and SN-38 covalently linked with a hydrolyzable CL2A linker, which was approved by the FDA in 2020 for the treatment of TNBC and metastatic uroepithelial cancer. The CL2A linker has a short PEG (polyethylene glycol) residue to aid in solubility and is coupled to SN-38 at the 20th position of the lactone ring, which stabilizes the ring from opening to the less active carboxylate form.
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Figure 3. Sacituzumab govitecan (IMMU-132) composition 
Other ADCs developed based on SN-38 include IMMU-130 (Labetuzumab govitecan) and IMMU-140, which target CEACAM5 and HLADR, respectively.
IMMU-130 demonstrated acceptable toxicity and activity in clinical Phase I studies; however, the phase II evaluation has been terminated in 2020 for undeclared reasons.
IMMU-140 is directed against HLA-DR and has shown promising preclinical activity both in hematological malignancies and melanoma. In vitro, IMMU-140 exhibits dual apoptotic signaling pathways: Anti–HLA-DR- and SN-38–mediated signals. In human tumor xenograft models of acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL), multiple myeloma (MM), acute myelogenous leukemia (AML), diffuse large B-cell lymphoma (DLBCL), Hodgkin's lymphoma (HL), and melanoma, IMMU-140 provided significant therapeutic efficacy when compared to controls. Among them, the therapeutic effect on MM and HL tumors was significantly better than that of the naked antibody, and these favorable preclinical results bode well for the potential clinical application of this drug.
Exatecan Derivatives Based ADCs
Building on the success of SN-38-based ADC drugs, a novel topoisomerase I inhibitor, Exatecan (DX-8951F), has been developed. The novel exatecan derivative, DXd, is 10 times more potent than the active form of irinotecan, SN-38, in inhibiting topoisomerase I. DXd was used in several proprietary ADC programs, such as DS-8201a (Enhertu®), U3-1402 and DS-6157a, conjugated at DAR8 and DS-1062a and DS-7300a conjugated at lower DAR (4) to limit their toxicity. Although the DXd payload presented lower passive membrane permeability than exatecan mesylate, it was found to be less myelotoxic and was therefore also selected for its improved safety profile.
Figure 4. Structure of DXd-based ADCs. 
Trastuzumab deruxtecan (DS-8201, T-DXd, Enhertu®) is composed of the already approved HER2-targeting antibody trastuzumab, attached to 8 DXd payloads through a maleimide-based mc-GGFG-am protease cleavable linker. Based on four clinical Phase 3 studies, trastuzumab deruxtecan was approved in 2019 for the treatment of unresectable metastatic HER2-positive breast cancer, approved for advanced and metastatic HER2-positive gastric cancer in 2021, approved for unresectable or metastatic HER2 low-expressing breast cancer in 2022 and approved for unresectable and metastatic HER2-positive non-small cell lung cancer (NSCLC) in 2022. T-DXd has demonstrated better efficacy than T-DM1 in patients with breast cancer and has also shown better activity than irinotecan in patients with gastric cancer, as well as good tolerability and benefit in patients with colorectal and gastric cancer.
Topoisomerase inhibitors are a novel class of therapeutic agents whose mechanism of action is to bind to the DNA-topoisomerase complex, causing DNA strand breaks and ultimately apoptosis. There are currently two main topoisomerase I inhibitors as payloads in ADCs in clinical trials:(1) SN-38 and (2) Exatecan derivative DXD.
It is expected that more ADCs utilizing topoisomerase I inhibitors as payloads will go through the regulatory approval process and become available to cancer patients, thereby increasing the options available to clinicians and patients.
 Han S, Lim KS, Blackburn BJ, Yun J, Putnam CW, Bull DA, Won YW. The Potential of Topoisomerase Inhibitor-Based Antibody-Drug Conjugates. Pharmaceutics. 2022 Aug 16;14(8):1707. doi: 10.3390/pharmaceutics14081707. PMID: 36015333; PMCID: PMC9413092.
 Conilh L, Sadilkova L, Viricel W, Dumontet C. Payload diversification: a key step in the development of antibody-drug conjugates. J Hematol Oncol. 2023 Jan 17;16(1):3. doi: 10.1186/s13045-022-01397-y. PMID: 36650546; PMCID: PMC9847035.
 Goldenberg DM, Sharkey RM. Antibody-drug conjugates targeting TROP-2 and incorporating SN-38: A case study of anti-TROP-2 sacituzumab govitecan. MAbs. 2019 Aug/Sep;11(6):987-995. doi: 10.1080/19420862.2019.1632115. Epub 2019 Jul 18. PMID: 31208270; PMCID: PMC6748572.
Camptothecin & Its Derivatives for Cancer Therapy
Overview of ADC-Based Combination Therapies
Advances in TROP-2 Directed ADCs
Summary of Approved HER2 ADCs on The Market & in Clinical Trials