DNA topoisomerase I (TOP1)

Overview

Topoisomerase 1 Dmel_CG6146 (Wikidata: Q29814717) is the Drosophila melanogaster ortholog of human DNA topoisomerase I (TOP1), encoded by CG6146. Topoisomerase I resolves the topological strain that accumulates ahead of replication forks and transcribing polymerases: it cleaves one strand of the duplex, forming a transient covalent intermediate — the TOP1 cleavage complex (TOP1cc), in which an active-site tyrosine is linked to the 3′ end of the nick — allows the DNA to rotate about the intact strand, and then reseals it. Unlike type II enzymes it needs no ATP, since the energy of the broken phosphodiester bond is stored in the protein–DNA link and recovered on religation. The reaction is essential in both directions of gene expression, and without it transcription generates positive supercoils ahead and negative supercoils behind that stall the polymerase and promote R-loop formation.

The enzyme is one of the most validated antitumor drug targets, for a reason that lies in its intermediate rather than its function. camptothecin-class inhibitors do not block catalysis; they bind at the enzyme–DNA interface and prevent religation, converting a normal transient intermediate into a persistent protein-linked single-strand break. Double-strand breaks arise secondarily, when a replication fork or transcription complex collides with the trapped complex or during its processing — which makes the drugs S-phase selective and makes exposure time, rather than peak concentration, the determinant of cell kill. Cells defend themselves by removing the adduct: tyrosyl-DNA phosphodiesterase 1 (TDP1) hydrolyzes the tyrosyl–DNA bond, and its loss sensitizes cells to these agents, while hereditary TDP1 deficiency causes a neurodegenerative ataxia, showing that trapped complexes accumulate in non-dividing neurons too.

The Drosophila protein is highly conserved with its human counterpart and has served as a tractable genetic and biochemical model for TOP1 function, inhibitor sensitivity and the DNA damage response — a role that depends on that conservation, since the trapping mechanism and its repair are shared across the eukaryotes.

Recent Publications Summary

Recent studies on DNA topoisomerase I (TOP1) have focused heavily on developing new small-molecule inhibitors and payload platforms with improved potency, selectivity, and drug-like properties. Several medicinal chemistry programs reported novel heterocyclic scaffolds with TOP1 inhibitory activity, including flavonolactam derivatives, fused pyridopyrimidines, benzothiophene/benzothienopyran analogs, sophoridine- and matrine-based triphenylpyridine derivatives, and anthrafuran analogs. Across these studies, TOP1 inhibition was linked to cell-cycle arrest, apoptosis, and broad antitumor activity in multiple cancer models, with some compounds showing activity comparable to camptothecin or doxorubicin and, in some cases, reduced toxicity toward non-cancerous cells 41934689Apr41922123Apr41844052Mar41812411Mar41905253Mar.

A major theme was overcoming the limitations of TOP1-targeted therapeutics through improved formulation or dual-function design. One study reprogrammed aromatic camptothecins into TOP1 degraders by combining hydrophobic tagging with supramolecular self-assembly, producing proteasome-dependent degraders with enhanced membrane permeability and pH-responsive nanoparticle formation; the lead compound SN-38-A2 showed potent TOP1 degradation and superior tumor regression versus irinotecan in xenograft models 42378337Jun. Another report described bifunctional SN-38–nitrogen mustard conjugates, where salt-form and phosphate modification improved solubility and the resulting molecules inhibited TOP1, promoted its degradation, and induced DNA crosslinking, yielding broad antiproliferative activity 41865567Mar. In an ADC context, a topoisomerase I inhibitor-based antibody-drug conjugate was used to examine analytical challenges in drug-to-antibody ratio determination, highlighting thermal hydrolysis and lactone ring opening as important factors affecting payload stability and chromatographic accuracy 42237689Jun.

TOP1-targeted antibody-drug conjugates were also examined in disease-relevant resistance settings. In advanced colorectal cancer, the anti-TROP2 ADC IMMU132 was shown to deliver SN-38 to induce TOP1-mediated DNA damage and cytotoxicity, while also suppressing the PERK-eIF2α-ATF4 stress-response axis; combining IMMU132 with the PERK inhibitor GSK2606414 produced synergy in preclinical models and further suppressed Wnt/β-catenin signaling 42030933Apr. A separate ADC study described ZW191 as an FRα-targeted topoisomerase 1 inhibitor ADC with a differentiated antitumor efficacy and tolerability profile, underscoring continued interest in TOP1 payloads for targeted delivery 42007996Apr.

Resistance to TOP1-directed therapy was also addressed through combination strategies that interfere with DNA repair. In cervical cancer, a newly synthesized TDP1 inhibitor, 16b, showed potent inhibition of tyrosyl-DNA phosphodiesterase 1 and synergized with topotecan to suppress HeLa cell proliferation, enhance DNA damage, induce apoptosis and S-phase arrest, and inhibit tumor growth in xenograft models 42012226Apr. Together, these publications indicate that recent TOP1 research spans direct inhibitor discovery, degrader engineering, ADC payload optimization, and combination approaches designed to intensify TOP1-mediated DNA damage while counteracting resistance mechanisms 42378337Jun42030933Apr42012226Apr42237689Jun.

What Changes, What Holds

1. TOP1 remains a tractable target, but the recent chemistry mainly extends the inhibitor class rather than revising TOP1 biology
REINFORCES New heterocyclic series broaden the medicinal chemistry around DNA topoisomerase I without changing the baseline account that TOP1 is a validated antitumor target whose inhibition traps TOP1cc and drives DNA damage. The main implication is practical: the field is still finding ways to tune potency, selectivity, and tolerability, not redefining the enzyme’s core role. The reported antiproliferative and pro-apoptotic effects fit the established model 41934689Apr41922123Apr.

2. TOP1 can now be viewed as a degradable payload as well as an enzymatic target, but that does not displace the cleavage-complex model
NEW DIRECTION Reprogramming camptothecin chemistry toward TOP1 degradation adds a distinct therapeutic strategy that the Overview does not cover: instead of only stabilizing TOP1cc, these compounds aim to remove TOP1 protein and exploit formulation behavior for delivery. That widens the drug concept around TOP1, while the bifunctional conjugates still rely on the same target biology and DNA damage logic. The analytical ADC work is methodological, highlighting payload stability issues rather than changing mechanism 42378337Jun41865567Mar.

3. TOP1 payloads remain central to ADC design, and resistance biology now points to stress signaling as a co-target
REINFORCES The ADC studies strengthen the established view of TOP1 as a useful cytotoxic payload for targeted delivery, especially in tumors where direct TOP1 damage can be coupled to additional pathway suppression. What changes is not the target’s basic role, but the understanding that efficacy may depend on how TOP1-mediated DNA damage intersects with unfolded-protein and Wnt/β-catenin stress responses, and that FRα or TROP2 targeting can shape tolerability and activity 42030933Apr42007996Apr.

4. Blocking repair of TOP1 lesions emerges as a rational way to intensify topotecan response
NEW DIRECTION Inhibiting TDP1 does not contradict the Overview; it extends it by showing that TOP1-directed therapy can be potentiated upstream of apoptosis by preventing repair of TOP1-linked DNA damage. That makes resistance management part of the TOP1 story, not just inhibitor potency. The key implication is combinatorial: TOP1 remains the primary lesion generator, but repair suppression may determine whether that lesion becomes lethal in cervical cancer models 42012226Apr.

Overview update candidates: TOP1 degraders as a new therapeutic modality; TOP1-targeted ADCs as established payload platforms with resistance/stress-signaling implications; TDP1 inhibition as a combination strategy to potentiate TOP1 therapy.