Topoisomerase I-Targeting Drugs

Alex Sparreboom

Ken-ichi Fujita

William C. Zamboni

DNA Topoisomerase I

DNA topoisomerases are essential enzymes found in all nucleated cells. These enzymes are involved in the regulation of DNA topology and are necessary for the preservation of the integrity of the genetic material during DNA metabolism, including RNA transcription, DNA replication, recombination, chromatin remodeling, chromatin condensation, and repair during cell division.¹ Based on their different reaction mechanism and cellular function, there are two types of DNA topoisomerases (type I and type II). Human topoisomerase I is a monomer, approximately 91-kDa polypeptide of 765 amino acids encoded by an active copy gene located on chromosome 20q12-13.2.² The protein is comprised of four major domains: a highly charged NH₂-terminal domain, a conserved core domain, a positively charged linker domain, and a highly conserved COOH-terminal domain containing the active site tyrosine, that is, the nucleophilic Tyr⁷²³ amino acid residue.³

The stabilization of the cleavable complexes by topoisomerase I inhibitors disrupts the DNA integrity and interferes with the normal processes of DNA topology, including replication, transcription, DNA repair, chromosome condensation, and chromosome separation.⁴ The formation of these cleavable complexes is essential but is not sufficient in itself to cause cytotoxicity, implying that cells need to undergo DNA synthesis to yield maximum toxicity. Stabilization of the cleavable complex, for example by camptothecins, is not sufficient in itself for the induction of cell death because the complex can reverse spontaneously. The lethal effects of these drugs are caused by the interaction between a moving replication fork and the drug-stabilized cleavable complex, resulting in irreversible arrest of DNA replication and the formation of a double-strand break located at the fork. A cytotoxic mechanism for camptothecins has been suggested recently, in which the accumulation of positive supercoils ahead of the replication machinery induces potentially lethal DNA lesions.⁵ Currently, the camptothecins remain the best-characterized inhibitors of topoisomerase I, and two agents in this class, topotecan (Hycamtin) and irinotecan (Camptosar), have been approved for human use.

Topotecan

Topotecan is a water-soluble camptothecin derivative containing a stable basic side chain at position 9 of the A ring of 10-hydroxycamptothecin (Fig. 17-1). Clinical trials of topotecan were initiated in 1989; it was approved for use as second-line chemotherapy in patients with advanced ovarian cancer in 1996, and for the treatment of small cell lung cancer (SCLC) after failure of initial or subsequent chemotherapy in 1998. Key features of topotecan are listed in Table 17-1.

Clinical Pharmacology

The most common dose and schedule of topotecan administration is a 30-minute IV infusion of 1.5 mg/m² daily for 5 days every 3 weeks.⁶ This regimen has undergone the most widespread clinical testing, and it is currently the dosage of topotecan approved for treating ovarian and lung cancer. Five-day continuous infusions of topotecan at 2.0 mg/m²/d have been tested in patients with hematologic malignancies, although in these studies gastrointestinal toxicities such as mucositis and diarrhea became more problematic. Prolonged 21-day infusion schedules at 0.5 to 0.6 mg/m²/d have been disappointing in phase II studies. Other schedules tested in phase I or phase II studies include a single 30-minute infusion, 24-hour infusions, and 3-, 5-, and 14-day continuous infusions. Oral administration has also been tested clinically and has been compared with IV administration in randomized studies.⁷, ⁸, ⁹, ¹⁰, ¹¹ Furthermore, IP, intrathecal, and individual adaptive, pharmacokinetically guided topotecan studies have been completed.

General Pharmacokinetics

After IV topotecan administration, the lactone ring undergoes rapid hydrolysis to generate the carboxylate species. Less than 1 hour after the start of an infusion, the majority of the circulating drug in plasma is in the carboxylate form, and this species predominates for the duration of the monitoring period. In most studies, the ratio of the lactone to total topotecan area under the concentration versus time curve (AUC) ranged from 20% to 35%. Interindividual variation in the AUC and the total-body clearance is quite large for both lactone and total topotecan (lactone plus carboxylate). In general, plasma concentrations and AUC levels tended to increase with increasing dose levels, consistent with linear pharmacokinetics, although in some studies, nonlinearity in drug clearance at higher dose levels was seen.¹²

For topotecan lactone, the terminal half-life ranges from 2.0 to 3.5 hours, which is relatively short compared with that of other camptothecin analogs. Consequently, no accumulation of drug is observed when administered daily for five consecutive days. Population pharmacokinetic studies in patients treated with IV or orally administered topotecan revealed that patient characteristics (i.e., gender, height, weight) and laboratory values (i.e., serum creatinine
concentration) give a moderate ability to predict the clearance of topotecan in an individual patient.¹³,¹⁴

FIGURE 17-1 Base structure of camptothecin analogues showing the lactone (left) and ring-opened carboxylate forms (right).

Absorption

The most common route for topotecan administration has been IV; however, oral formulations using prolonged administration schedules have undergone preclinical and clinical testing. In humans, the reported oral bioavailability ranged from 30% to 42%¹⁵ and is influenced by multiple factors. First, the relatively high pH in the small bowel leads to conversion to the carboxylate form, which is poorly absorbed by the intestinal walls. Second, the bioavailability is reduced by protein-mediated, outward-directed transport of topotecan by ABCG2.¹⁶ Third, the bioavailability is partly influenced by the binding of topotecan to food, proteins, and intestinal fluids and/or by decomposition in the gastrointestinal fluid. From a clinical point of view, as long as equivalent safety and efficacy can be ensured, the majority of patients prefer oral instead of IV administration of chemotherapy.¹⁷

TABLE 17.1 Key features of topotecan

Mechanism of action	Topoisomerase I poison. Stabilizes the cleavable complex in which topoisomerase I is covalently bound to DNA at a single-stranded break site.
Metabolism	Nonenzymatic hydrolysis of the lactone ring generates the less active open-ring hydroxy carboxylic acid. N-desmethyl is a minor metabolite.
Elimination	About 26%-41% excreted unchanged in urine over 24 h. Concentrated in the bile at levels that are 1.5 times higher than the simultaneous plasma levels.
Pharmacokinetics	Terminal half-life of topotecan lactone is 3 h; approximate clearance of 62 L/h/m² (range, 14-155 L/h/m²) reported for 30-min topotecan infusions.
Toxicity	Myelosuppression, predominantly neutropenia with thrombocytopenia
	Nausea and vomiting (mild)
	Diarrhea (mild)
	Fatigue
	Alopecia
	Skin rash
	Elevated liver function test results
	Mucositis
Modifications for organ dysfunction	In minimally pretreated patients, no dosage adjustments appear to be necessary for patients with mild renal impairment (creatinine clearance 40-60 mL/min), but dosage adjustment to 0.75 mg/m²/d is recommended for patients with moderate renal impairment (20-39 mL/min). Further dosage adjustments may be necessary for patients with extensive prior chemotherapy or radiation therapy. Dosage adjustments are not required for hyperbilirubinemia up to 10 mg/dL.
Precautions	For febrile or severe grade 4 neutropenia lasting >3 d, the dosage for subsequent courses should be reduced by 0.25 mg/m²/d. Monitoring of blood counts is essential.

Metabolism

An N-desmethyl metabolite of topotecan has been characterized and found to be present at relatively low concentrations in human plasma, urine, and feces after IV administration of topotecan.¹⁸ Although a specific interaction with drug metabolism enzymes has not been established, clinical data suggest enhanced topotecan clearance in pediatric patients simultaneously receiving treatment with agents that induce hepatic cytochrome P-450 (CYP) enzymes, such as dexamethasone, phenobarbital, and phenytoin.¹⁹ These observations are consistent with a potential drug interaction at the level of CYP3A; however, additional studies on the metabolism and excretion of topotecan and its potential for drug interactions are warranted.

Topotecan and its main metabolite can also undergo further metabolism into a UGT-mediated glucuronide product.²⁰ This is a reversible transformation because β-glucuronidase, actively found in intestinal bacteria, is able to reform topotecan. Because topotecan is metabolized in the liver only to a minor extent, it is not surprising that the pharmacokinetics in patients with impaired liver function did not significantly differ from those in patients with normal hepatic function.²¹ In contrast, patients with moderately impaired renal function had significantly reduced plasma clearance.²² As a consequence, dose modifications are recommended for patients with impaired renal function and are not required for patients with liver dysfunction.

Excretion

Elimination of topotecan lactone is thought to result mainly from the rapid hydrolysis to the carboxylate species followed by renal excretion of the open-ring metabolite. Overall, 25% to 49% of the dose administered is excreted in the urine over a 24-hour period,²³ a few pediatric reporting recovery of over 90% of the administered drug during more prolonged periods of urine collection.²⁴

General Pharmacodynamics

Pharmacodynamic correlations between parameters of systemic drug exposure and topotecan drug effects have been observed inconsistently.²⁵ In a pediatric study, the topotecan total (lactone plus carboxylate) AUC and lactone plasma AUC correlated with the percentage change in the platelet count and the granulocyte count after a 72-hour drug infusion.²⁶ Similar correlations between leukocyte or granulocyte counts and total topotecan AUC or topotecan dose have been reported after a 30-minute infusion of topotecan given daily for 5 days, after 20-minute infusions every 3 weeks, and after 24-hour continuous infusions. In each case, measurement of the total topotecan plasma concentration was as informative about drug effects as the lactone drug levels. Thus, the need to measure lactone drug concentrations has been questioned by some investigators. Finally, although interpatient variability in topotecan plasma concentrations is high, the total variability in hematologic toxicity in patients is relatively low when the drug is administered according to standard dosing guidelines. Thus, therapeutic drug monitoring is not useful for this agent.

Adverse Reactions

Dose-related, reversible, and noncumulative myelosuppression is the most important side effect of topotecan.²⁷ Neutropenia occurs more frequently and is often more severe than thrombocytopenia, and it is more severe in heavily pretreated patients than in minimally pretreated patients. Besides myelosuppression, stomatitis (24% to 28% of patients) and late-onset diarrhea (40%) were noted at higher doses. Other nonhematological toxicities reported include alopecia, nausea, vomiting, fatigue, and asthenia.

Hematologic toxicity is more pronounced with the shorter oral regimens but is still mostly mild and noncumulative, whereas diarrhea is a severe and intractable side effect of more prolonged daily administration.²⁸ An analysis of the pharmacokinetic-pharmacodynamic relationships revealed that the total area under the curve per course did not differ between the various regimens and that the daily times five schedule provided the best systemic exposure and toxicity profile.²⁹

In acute leukemia, the maximum tolerated dose (MTD) of a daily 30-minute IV infusion for five consecutive days every 3 weeks was 4.5 mg/m²/d.³⁰ The dose-limiting toxicity (DLT) at higher dose levels was a complex of symptoms, consisting of high fever, rigors, precipitous anemia, and hyperbilirubinemia, which are possibly related to an acute hemolytic reaction.

Antitumor Activity

The antitumor activity of topotecan, given as a single agent using various schedules of administration, was established in a variety of phase II studies, including ovarian cancer, SCLC, NSCLC, breast cancer, myelodysplastic syndrome, and chronic myelomonocytic leukemia. Marginal activity was seen in head and neck cancer, prostate cancer, pancreatic cancer, gastric cancer, esophageal carcinoma, hepatocellular carcinoma, and recurrent malignant glioma, as well as when topotecan was used as consolidation treatment after first-line standard chemotherapy for ovarian cancer.³¹

In a phase III study, the daily times five IV topotecan was compared with paclitaxel (3-hour infusion of 175 mg/m²/d every 3 weeks) in ovarian cancer. In this disease, topotecan and paclitaxel were equally effective with regard to response rates, progression-free survival, and overall survival.³² In an open-label, multicenter study comparing the activity and tolerability of oral versus IV topotecan in patients with relapsed epithelial ovarian cancer after failure of one platinum-based regimen,⁷ oral doses of topotecan were administered as 2.3 mg/m²/d and IV doses as 1.5 mg/m²/d for five consecutive days every 3 weeks. No difference in response rates between the two treatment arms was reported. Although a small, statistically significant difference in survival favored the IV formulation, in the context of second-line palliative treatment for ovarian cancer, this difference in outcome has only limited clinical significance.⁷ For this reason, oral topotecan could be an alternative treatment modality in this setting because of its convenience and good tolerability.⁹, ¹⁰, ¹¹,³³

Another phase III study compared single-agent topotecan with combination chemotherapy consisting of cyclophosphamide, doxorubicin, and vincristine (CAV) in patients with SCLC relapsing after first-line platinum-based chemotherapy.³⁴ Although the response rate, time to disease progression, and overall survival were similar, the palliation of disease-related symptoms was better with topotecan. In a randomized trial performed by the Eastern Cooperative Oncology Group (ECOG), topotecan was compared with best support of care in patients with extensive SCLC. In this trial, topotecan was administered as consolidation therapy after response induction with cisplatin and etoposide.³⁵ Although topotecan produced a moderate increase in the time to disease progression, it did not improve survival. Finally, similar to the study of ovarian cancer, oral topotecan was compared to IV administration of topotecan in patients with relapsed and chemosensitive SCLC. The oral formulation was similar in efficacy, resulted in less severe neutropenia, and was more convenient.⁸ Although topotecan has shown some activity against hematological malignancies and advanced cervical cancer,³⁶,³⁷ its use for these specific indications is not clearly established.

Irinotecan

Irinotecan (CPT-11) (Fig. 17-2) was the first camptothecin derivative with increased aqueous solubility to enter clinical trials and
became commercially available in Japan for treatment of lung cancer, cervical cancer, and ovarian cancer in 1994. In the United States, irinotecan was approved in 1996 for use in patients with advanced colorectal cancer refractory to 5-FU, and in 2000, it was approved as a component of first-line therapy in combination with 5-FU/LV for the treatment of metastatic colorectal cancer or for patients who have progressed following initial 5-FU-based chemotherapy. Key features of irinotecan are listed in Table 17-2.

FIGURE 17-2 Metabolic pathways of irinotecan (CPT-11).

Clinical Pharmacology

Systemic exposure to irinotecan can vary up to 10-fold among patients receiving standard doses.³⁸,³⁹ Pharmacokinetic and pharmacodynamic properties of irinotecan can be affected by a variety of factors, including inherited genetic variability (see Chapter 24), age, sex, malnutrition, polypharmacy, complex physiological changes due to concomitant disease, organ dysfunction, and tumor invasion.

General Pharmacokinetics

Irinotecan is unique among camptothecin analogs in that it must first be converted by a carboxylesterase-converting enzyme to the active metabolite SN-38 (Fig. 17-2).⁴⁰ SN-38 is the major metabolite believed to be responsible for irinotecan’s biologic effects, including efficacy and toxicity. SN-38 is predominantly detoxified by a drug-metabolizing UDP-glucuronosyl transferase (UGT) 1A1 to form SN-38 glucuronide (SN-38G).⁴⁰ The relative AUC value of the active metabolite SN-38 to irinotecan varied from 0.9% to 11%. Irinotecan is also a substrate for metabolism by the CYP system, which creates an additional potential for drug interactions. Collectively, these studies demonstrate that the metabolic pharmacokinetics of irinotecan is complex and may be mediated by several different families of enzymes.

Carboxylesterase-Mediated Metabolism

Carboxylesterase-converting enzyme-specific activity is much lower in human serum and in comparable human tissues than that in rodents.⁴¹ The main carboxylesterase responsible for the clinical activation of irinotecan in humans is carboxyl esterase 2 (CES2).⁴² Carboxylesterase activity in human liver is found in the microsomal fractions, and this enzyme has been cloned and characterized.⁴¹ In human and animal studies, no evidence exists that irinotecan induces hepatic or serum carboxylesterase activity.⁴³

In most but not all studies, irinotecan and SN-38 plasma concentrations and AUC increased proportionally with increasing dose, which suggests linear pharmacokinetics.⁴⁰ The plasma half-life of SN-38 is relatively long compared with that of the other camptothecins, approximately 10 hours. The prolonged duration of exposure to SN-38 is probably a function of its sustained production from
irinotecan in tissues by CES2 because direct injection of SN-38 into rats resulted in extremely rapid plasma clearance, with a half-life of only 7 minutes.⁴⁴

TABLE 17.2 Key features of irinotecan

Mechanism of action	After metabolic activation to SN-38, the mechanism of action is the same as for topotecan.
Metabolism	Irinotecan is a prodrug that requires enzymatic cleavage of the C-10 side chain by an irinotecan carboxylesterase-converting enzyme to generate the biologically active metabolite SN-38. Irinotecan can also undergo hepatic oxidation of its dipiperidino side chain to form the inactive metabolite 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino]carbonyloxycamptothecin (APC).
Elimination	Elimination of irinotecan occurs by urinary excretion, biliary excretion, and hepatic metabolism. About 16.1% (range, 11.1%-20.9%) of an administered dose of irinotecan is excreted unchanged in the urine. SN-38 is glucuronidated, and both the conjugated and unconjugated forms are excreted in the bile.
Pharmacokinetics	Approximate terminal half-life of irinotecan lactone is 6.8 h (range, 5.0-9.6 h) and approximate clearance is 46.9 L/h/m² (range, 39.0-53.5 L/h/m²). Approximate terminal half-life of SN-38 lactone is 11 h (range, 9.1-13.0 h).
Toxicity	Early-onset diarrhea within hours or during the infusion is associated with cramping, vomiting, flushing, and diaphoresis. Consider atropine 0.25-1.0 mg SC or IV in patients experiencing cholinergic symptoms. Late-onset diarrhea can occur later than 12 h after drug administration.
	Myelosuppression, predominantly neutropenia
	Alopecia
	Nausea and vomiting
	Mucositis
	Fatigue
	Elevated hepatic transaminases
	Pulmonary toxicity (uncommon) associated with a reticulonodular infiltrate, fever, dyspnea, and eosinophilia
Modifications for organ dysfunction	No definite recommendations are available for patients with impaired renal or hepatic dysfunction. Caution is warranted in patients with Gilbert’s disease.
Precautions	Severe delayed-onset diarrhea may be controlled by high-dose loperamide given in an initial oral dose of 4 mg followed by 2 mg every 2 h during the day and 4 mg every 4 h during the night. High-dose loperamide should be started at the first sign of any loose stool and continued until no bowel movements occur for a 12-h period. Particular caution is also warranted in monitoring and managing toxicities in elderly patients (>64 y) or those who have previously received pelvic/abdominal irradiation.

UGT-Mediated Metabolism of SN-38

The major metabolite of SN-38 is a glucuronidated derivative SN-38G, which is present in the plasma and bile of patients receiving irinotecan chemotherapy.⁴⁰ The decrease in plasma concentrations of SN-38G tends to parallel the decrease in SN-38 over time, suggesting that UGT is the rate-limiting step responsible for the elimination of SN-38.

The UGT1A isoforms UGT1A1, UGT1A3, UGT1A6, UGT1A7, and UGT1A9 have all been implicated in the glucuronidation of SN-38,⁴⁵ although UGT1A1 is believed to be predominantly responsible for SN-38 metabolism in humans.⁴⁶,⁴⁷ UGT1A1 is known to be a highly polymorphic enzyme. Lines of evidence show that some of polymorphic variants are related to variability in the pharmacokinetics and toxicity (especially neutropenia in the every 3 week schedule) associated with irinotecan⁴⁸, ⁴⁹, ⁵⁰, ⁵¹ and that enzyme activity can be modulated by some prescription drugs such as ketoconazole⁵² (see Chapter on “Pharmacogenetics”).

CYP3A-Mediated Metabolism

Several additional metabolites of irinotecan have been identified and characterized in human matrices. They result from oxidation of the terminal piperidino ring.⁵³ CYP3A4 is thought to be responsible for the main oxidative irinotecan metabolites, known as APC and NPC.⁴⁰ APC is at least 100-fold less active than SN-38 as an inhibitor of topoisomerase I and is a poor substrate for conversion to SN-38 by human liver carboxylesterases. Nonetheless, formation of APC may represent an important metabolic pathway for irinotecan clearance.⁵⁴,⁵⁵

Transporter-Mediated Excretion

In addition to hepatic metabolism, elimination of irinotecan also occurs by urinary and fecal excretion. Up to 37% of the administered irinotecan dose is excreted unchanged in the urine over 48 hours after a short 90-minute infusion, with only less than 0.3%
being excreted as SN-38.⁵⁶ Biliary secretion of irinotecan, SN-38, and SN-38G also contributes substantially to drug elimination. The canalicular multispecific organic anion transporter ABCC2 (cMOAT; MRP2) is believed to be responsible for the biliary secretion of irinotecan carboxylate, SN-38 carboxylate, and the carboxylate and lactone forms of SN-38G.⁵⁷ SN-38 is also a substrate for other transport systems in the bile canaliculi such as ABCC1 (MRP1) and ABCG2 (BCRP; MXR), but not ABCB1.⁴⁰ OATP1B1 (OATP-C), which transports a variety of drugs and their metabolites from blood into hepatocytes, showed transport activity for SN-38 but not for irinotecan and SN-38G.⁵⁸

Theoretically, another way of reducing gastrointestinal toxicity is by blocking biliary excretion of SN-38 and SN-38G. Cyclosporin A can reduce bile flow and inhibit bile canalicular active transport, and thus it is a potential modulator of SN-38-induced toxicity. Coadministration of cyclosporin A with irinotecan in rats increased the AUCs of irinotecan, SN-38, and SN-38G by 3.4-, 3.6-, and 1.9-fold, respectively,⁵⁹ suggesting this might be a possible strategy for reducing the gastrointestinal toxicities of irinotecan.

General Pharmacodynamics

Based on preclinical studies, it is likely that clinical outcome to irinotecan treatment might be altered by the three main mechanisms: (a) alterations in the target (topoisomerase I), (b) changes in the accumulation of drug in the tumor cells, and/or (c) alterations in the cellular response to the topoisomerase I.⁶⁰

Alterations in Topoisomerase I

Various point mutations of topoisomerase I in different camptothecin-resistant cell lines have been associated with camptothecin resistance.⁶¹ These point mutations result in decreased topoisomerase I catalytic activity or impaired binding of camptothecin to topoisomerase I. In some models, single amino acid changes resulted in partial resistance, while double mutation induced a synergistic resistance.

In clinical studies, point mutations were identified in patients treated with irinotecan that were located near a site in topoisomerase I previously identified as a position of a mutation in a camptothecin-resistant human lung cancer cell line.⁶² In addition, amplification of topoisomerase I occurs in greater than 20% of colorectal cancers and was found to be associated with higher RNA and protein expression levels than the diploid tumors. This study suggests a potential pharmacogenomic influence of topoisomerase I copy-number alteration on its RNA/protein expressions.⁶³,⁶⁴

Altered Cellular Accumulation

The role of ABCB1-associated multidrug resistance (MDR) phenotypes in camptothecin resistance has still not been clearly defined. Nonetheless, irinotecan and SN-38 do not appear to interact significantly with ABCB1, and cross-resistance to irinotecan is not seen in P388 leukemia cells expressing pleiotropic drug resistance to vincristine and doxorubicin.⁶⁵

In addition to active transport, cellular metabolism may be particularly important for irinotecan.⁶⁶ Indeed, increased levels of esterases are associated with increased sensitivity to irinotecan.⁶⁷ A large interindividual variation in expression of carboxylesterases in colon tumors may contribute to the variability of outcome of irinotecan therapy.⁶⁸,⁶⁹

Alternative Mechanisms

Other potential mechanisms of decreased sensitivity to camptothecins include a reduction in the number of cells in the S phase and increased expression of metallothionein.⁷⁰ Furthermore, double-stranded DNA break repair (DSBR) activity may also modulate camptothecin-induced cytotoxicity. For example, yeast mutants defective in the RAD52 DSBR gene are hypersensitive to camptothecin.⁷¹,⁷² Whether up-regulated DSBR leads to resistance is not known.

Also important, but even less well understood, is the role of events downstream from the formation of cleavable complexes, such as DNA damage repair, the triggering of apoptosis, and alterations in the integrity of the G₂

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