the formation of reversible CPT-induced Top 1-DNA cleavable complex. In particular, CPT attaches to the Top 1-DNA complex and forms the reversible Top 1–
CPT–DNA covalent tertiary complexes that inhibit the Top 1-religation reaction
(Redinbo et al. 1998; Zhang et al. 2011; Das et al. 2016). Further, it was shown that
CPT-induced cytotoxicity is directly related to the CPT-mediated assembly of Top
1-DNA cleavable complexes (Hsiang and Liu 1988; Hsiang et al. 1989; Liu et al.
2000; Pommier 2006). The interaction of CPT with the DNA occurs by intercalating at the cleavage site of the enzyme. Also, it has been suggested that other
interactions might occur between CPT and Top 1, as well as CPT and a flipped base
of DNA at the +1 site (Liu et al. 2000). However, these molecular interactions at
Top 1-CPT-DNA complex are yet to be understood in detail. The cell cytotoxicity
effect of CPT is mostly mediated during S-phase (synthetic phase) of the cell cycle
(Cliby et al. 2002; Pommier 2006; Raveendran 2015). Based on the understanding
of S-phase-specific cytotoxicity of CPT, researchers have proposed a replication
fork collision model. According to this model, Top 1-CPT-DNA tertiary complexes
are reversible and non-lethal by themselves but, after colliding with the replication
forks causes DNA strand break, leading to apoptosis (Hsiang et al. 1989; Liu et al.
2000; Pommier 2006). After the collision of replication fork, the following events
are observed: (1) double-strand breakage, (2) replication fork drive arrest, and
(3) the establishment of Top 1 induced DNA breaks at the collision site. However,
these biomolecular interactions leading to cell toxicity are yet to be understood in
detail (Liu et al. 2000; Raveendran 2015). Interfacial inhibitor concept has been
critically reviewed for CPT by Pommier (2006) and Pommier (2009). Accordingly,
CPT binds at the Top 1-DNA interface and trap cleavage complexes. It is proposed
that these topoisomerase inhibitor drugs stack between the DNA base pairs
adjoining the cleavage site due to their aromatic nature (Pommier 2009; Koster
et al. 2007; Pommier and Marchand 2012). CPT obstructs the rotation of DNA after
intercalating at the enzyme–DNA complex through the interaction of p-p electrons
between the nucleotide bases flanking the cleavage site and the aromatic structure
of the drug (Marchand et al. 2006; Koster et al. 2007). This interference formation
inhibits the release of torsional stress and when the replication fork progresses
further, it collides with Topo 1-CPT-DNA tertiary complex inducing DNA strand
breakage and cell death. CPT, at higher concentrations can also destroy
S-phase-independent cells through transcriptionally mediated DNA damage and
apoptosis (Morris et al. 1996). Another prominent way of CPT-induced cell toxicity
is by arresting RNA synthesis during transcription elongation process (Ljungman
et al. 1996; Liu et al. 2000). It is also reported that the Top 1 cleavable complex is
attached by ubiquitin/26S proteasome complex that blocks the re-ligation step of
the Top 1 reaction (Desai et al. 1997; Liu et al. 2000). CPT, a Top 1-specific poison
induces the attachment of SUMO-1 (Small Ubiquitin-like MOdifier) to Top 1 and
facilitate its degradation (Mao et al. 2000; Rallabhandi et al. 2002). However, the
exact function of SUMOylation of Top 1 in response to CPT is not clear. It is
believed that SUMO-1 regulates the cellular localization of Top 1 as it contains
nuclear localization signals (Desai et al. 1997, 2001; Mo et al. 2002). On the other
hand, Poly(ADP-ribose) polymerase-1 (PARP 1), a chromatin-associated enzyme
9 Camptothecin: Occurrence, Chemistry and Mode of Action
321
CPT–DNA covalent tertiary complexes that inhibit the Top 1-religation reaction
(Redinbo et al. 1998; Zhang et al. 2011; Das et al. 2016). Further, it was shown that
CPT-induced cytotoxicity is directly related to the CPT-mediated assembly of Top
1-DNA cleavable complexes (Hsiang and Liu 1988; Hsiang et al. 1989; Liu et al.
2000; Pommier 2006). The interaction of CPT with the DNA occurs by intercalating at the cleavage site of the enzyme. Also, it has been suggested that other
interactions might occur between CPT and Top 1, as well as CPT and a flipped base
of DNA at the +1 site (Liu et al. 2000). However, these molecular interactions at
Top 1-CPT-DNA complex are yet to be understood in detail. The cell cytotoxicity
effect of CPT is mostly mediated during S-phase (synthetic phase) of the cell cycle
(Cliby et al. 2002; Pommier 2006; Raveendran 2015). Based on the understanding
of S-phase-specific cytotoxicity of CPT, researchers have proposed a replication
fork collision model. According to this model, Top 1-CPT-DNA tertiary complexes
are reversible and non-lethal by themselves but, after colliding with the replication
forks causes DNA strand break, leading to apoptosis (Hsiang et al. 1989; Liu et al.
2000; Pommier 2006). After the collision of replication fork, the following events
are observed: (1) double-strand breakage, (2) replication fork drive arrest, and
(3) the establishment of Top 1 induced DNA breaks at the collision site. However,
these biomolecular interactions leading to cell toxicity are yet to be understood in
detail (Liu et al. 2000; Raveendran 2015). Interfacial inhibitor concept has been
critically reviewed for CPT by Pommier (2006) and Pommier (2009). Accordingly,
CPT binds at the Top 1-DNA interface and trap cleavage complexes. It is proposed
that these topoisomerase inhibitor drugs stack between the DNA base pairs
adjoining the cleavage site due to their aromatic nature (Pommier 2009; Koster
et al. 2007; Pommier and Marchand 2012). CPT obstructs the rotation of DNA after
intercalating at the enzyme–DNA complex through the interaction of p-p electrons
between the nucleotide bases flanking the cleavage site and the aromatic structure
of the drug (Marchand et al. 2006; Koster et al. 2007). This interference formation
inhibits the release of torsional stress and when the replication fork progresses
further, it collides with Topo 1-CPT-DNA tertiary complex inducing DNA strand
breakage and cell death. CPT, at higher concentrations can also destroy
S-phase-independent cells through transcriptionally mediated DNA damage and
apoptosis (Morris et al. 1996). Another prominent way of CPT-induced cell toxicity
is by arresting RNA synthesis during transcription elongation process (Ljungman
et al. 1996; Liu et al. 2000). It is also reported that the Top 1 cleavable complex is
attached by ubiquitin/26S proteasome complex that blocks the re-ligation step of
the Top 1 reaction (Desai et al. 1997; Liu et al. 2000). CPT, a Top 1-specific poison
induces the attachment of SUMO-1 (Small Ubiquitin-like MOdifier) to Top 1 and
facilitate its degradation (Mao et al. 2000; Rallabhandi et al. 2002). However, the
exact function of SUMOylation of Top 1 in response to CPT is not clear. It is
believed that SUMO-1 regulates the cellular localization of Top 1 as it contains
nuclear localization signals (Desai et al. 1997, 2001; Mo et al. 2002). On the other
hand, Poly(ADP-ribose) polymerase-1 (PARP 1), a chromatin-associated enzyme
9 Camptothecin: Occurrence, Chemistry and Mode of Action
321