Topotecan is a synthetic analog of the naturally occurring parent compound, camptothecin, which was originally derived from the Chinese tree, Camptotheca acuminata.[1-4] Topotecan contains both a C-9 tertiary amine side chain, a C-10 hydroxyl group that enhances its aqueous solubility, and a chiral carbon at C-20 (Figure 1). Like all camptothecin derivatives, the (S)-isomer is much more biologically active than the (R)-isomer. However, unlike the other clinically available camptothecin analog, irinotecan (Camptosar), topotecan is not a prodrug, and in the lactone form it can directly interact with topoisomerase I. An intact closed
lactone ring is essential for this activity, but the lactone also makes all camptothecins unstable in aqueous solutions by undergoing a rapid, reversible, pH-dependent, nonenzymatic hydrolysis to form the less active hydroxycarboxylic acid (Figure 1).[6,7] At neutral or basic pH, the equilibrium for this reaction favors the formation of the inactive topotecan carboxylate, while acidic pH favors the stabilization of topotecan in the active lactone form.
DNA topoisomerase I is a 100-kDa monomeric eukaryotic enzyme that relaxes torsionally strained, supercoiled, double-stranded DNA. Topoisomerase I activity is found in all mammalian cells and is potentially involved in DNA-related functions, such as replication, recombination, and RNA transcription. Topoisomerase I preferentially binds to both positively and negatively supercoiled duplex DNA, forming a short-lived catalytic intermediate in which the enzyme is covalently bound to a tyrosine residue at the 3´ end of a single-stranded DNA break. This normally transient intermediate, called the cleavable complex, allows for the relaxation of the torsional strain by facilitating passage of the intact strand through the single-stranded DNA nick, or by rotating around the remaining intact DNA phosphodiester bond. Rapid DNA relegation followed by enzyme dissociation regenerates a torsionally relaxed, intact double helix. Because of the rapidity of this reaction, topoisomerase I covalently bound to DNA in the cleavable complex is not normally detectable. However, in the presence of camptothecins, which inhibit the relegation reaction by binding noncovalently to the topoisomerase I-DNA cleavable complex, these protein-linked, single-strand DNA breaks accumulate within the cell (Figure 2). Not all topoisomerase I cleavage sites are equally stabilized by camptothecins. There is, instead, a preference for guanine residues at the +1 position relative to the single-stranded break.[12-13]
The persistence of topotecan-induced single-stranded DNA breaks is not sufficient to cause cell death, because drug removal results in regeneration of intact DNA. However, if a DNA replication fork encounters a topotecan-stabilized cleavable complex, these lesions convert into lethal double-stranded DNA damage (fork collision model; Figure 2).[14,15] This cytotoxic DNA damage in some cell lines can ultimately trigger pathways that lead to DNA degradation patterns consistent with programmed cell death, or apoptosis.[16,17] Because of the need for ongoing DNA synthesis, topotecan is most lethal during the S-phase of the cell cycle. This mechanism of action may have potential clinical consequences because cell-cycle, phase-specific cytotoxic agents typically require longer drug exposure times to maximize cell kill. Consequently, phase II evaluation of a 21-day continuous infusion of topotecan is nearing completion.[19-21]
Unlike antimetabolite agents, topotecan is not an enzyme inhibitor in the classic sense, because it does not cause cell death by eliminating the function of an essential enzyme. Instead, topotecan converts an endogenous enzyme, topoisomerase I, into a cellular poison. The presence of topoisomerase I activity is absolutely essential for drug activity. Higher levels of topoisomerase I have been reported in various human tumors, including leukemic blasts and colon, head and neck, and prostate cancers.[24,26] Because increased topoisomerase I activity can enhance sensitivity to camptothecins,[22,27] a potential selective advantage for killing these tumor cells was predicted for these drugs. However, more recent data have shown a poor correlation between absolute topoisomerase I protein levels and relative camptothecin sensitivity in various cultured cancer cell lines including ovarian, colon, and leukemic.[28-30]
Other factors also contribute to the induction of topotecan-induced cytotoxicity. Yeast mutants with decreased ability to repair double-stranded DNA damage are more sensitive to camptothecins than are wild-type cells, and more efficient repair of DNA double-strand breaks has been proposed as a potential mechanism of camptothecin resistance in human cells. Increased sensitivity to camptothecins by human leukemia cells has been correlated with loss of cell-cycle regulation and G2 checkpoint controlfactors that may allow cells to repair damaged DNA prior to replication. The relationships between camptothecin-induced cell death, cell-cycle regulation, DNA-repair, and the induction of apoptosis are currently being studied.
Although various mechanisms of topotecan resistance have been characterized in vitro, little is currently known about the mechanisms of clinical drug resistance. Cell lines with decreased topoisomerase I activity[34,35] or mutations in the topoisomerase I enzyme that preserve catalytic activity but diminish interactions with camptothecins[36-43] have demonstrated camptothecin resistance in laboratory experiments. Reduced intracellular drug accumulation may also contribute to drug resistance,[44,45] although the mechanisms of topotecan transport and retention within the cell have not been well characterized. The role of the P-glycoprotein-mediated, multidrug resistant (MDR) efflux pump in topotecan resistance is also undefined. Cultured cell lines that over express P-glycoprotein have less ability to accumulate intracellular drug and are about ninefold more resistant to topotecan.[46-48] However, this degree of resistance is much less than the 200-fold decrease in sensitivity seen with classic MDR substrates, such as doxorubicin or vinblastine. Hence, the relevance of these observations to the treatment of tumors that highly express MDR, such as colon cancer, must still be determined.
Camptothecin derivatives, such as topotecan, can cause chromosomal aberrations, including increased sister-chromatid exchanges, gene deletions, and gene rearrangements. Topoisomerase II inhibitors, such as doxorubicin and etoposide, which are known to increase the risk of secondary malignancies, such as acute myelogenous leukemia, cause similar patterns of DNA damage. The carcinogenic risk following topotecan therapy is unknown; however, the theoretical risk, which topotecan shares with other highly useful anticancer agents, should not deter its use in appropriate clinical settings. Nonetheless, following topotecan therapy, patients should be carefully monitored for adverse drug effects, particularly those patients with the potential for prolonged survival.
Topotecan may also be useful as a radiation sensitizer. Low concentrations of topotecan can enhance radiation lethality in laboratory experiments, and clinical trials are currently evaluating topotecan in combination with radiation therapy for several tumor types, including those in the lung and central nervous system.
Topotecan has been most commonly administered at the FDA-approved dose of 1.5 mg/m2/d infused daily over 30 minutes for 5 consecutive days every 3 weeks.[53-55] In hematologic malignancies, a 5-day continuous infusion at 2.0 mg/m2/d every 3 weeks has also been used.[23,56,57] Other schedules of administration evaluated in phase I trials include a single infusion every 3 weeks and continuous infusions for 24 hours,[59-61] 72 hours,[56,62] and 21 days. Oral and intraperitoneal topotecan administration has also been examined.
The most common toxicity of topotecan is myelosuppression, especially neutropenia, which is the dose-limiting effect of most schedules tested. Anemia and thrombocytopenia are less common; however, they may occur more frequently when topotecan is administered on weekly and 21-day infusion schedules. Although topotecan-induced myelotoxicity is not cumulative, heavily pretreated patients may be at increased risk for more severe cytopenias. In one phase I study, use of granulocyte-colony-stimulating factor (G-CSF [Neupogen]) did not allow further dose escalation because of development of dose-limiting thrombocytopenia and fatigue. Higher doses, however, up to 3.5 mg/m2/d, were administered in a second trial with G-CSF support. In preliminary results, when these higher, more toxic doses were given to patients with colorectal cancer, no substantial improvement in anticancer activity was noted. [E. K. Rowinsky, MD, personal communication, 1996]
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