The Current Status of Camptothecin Analogues as Antitumor Agents

Summary In order to unravel possible mechanisms of clinical resistance to topoisomerase inhibitors, we developed a topotecan-resistant human IGROV-1 ovarian cancer cell line, denoted IGROVTlOOr, by stepwise increased exposure to topotecan (TPT). The IGROVT100r cell line was 29-fold resistant to TPT and strongly cross-resistant to . However, the IGROVTlOOr showed only threefold resistance to camptothecin (CPT). Remarkably, this cell line was 32-fold resistant to mitoxantrone, whereas no significant cross-resistance against other cytostatic drugs was observed. No differences in topoisomerase protein levels and catalytic activity as well as topoisomerase cleavable complex stabilization by CPT in the IGROV-1 and IGROVTIOOr cell lines were observed, indicating that resistance in the IGROVTlOOr cell line was not related to topoisomerase I-related changes. However, resistance in the resistant IGROVTlOOr cell line to accompanied by decreased accumulation of the drugs to approximately 15% and 36% of that obtained in IGROV-1 respectively. No reduced accumulation was observed for CPT. Notably, accumulation of TPT in the IGROV-1 cell line decreased under energy-deprived conditions, whereas the accumulation in the IGROVTlOOr cell line increased under these energy-deprived conditions. The efflux of TPT at 370C was very rapid in the IGROV-1 as well as the IGROVT100r cell line, resulting in 90% efflux within 20 min. Importantly, the efflux rates of TPT in the IGROV-1 and IGROVTlOOr cell lines were not significantly different and were shown to be independent on P-glycoprotein (P-gp) or multidrug resistance-associated protein (MRP). These results strongly suggest that the resistance of the IGROVTlOOr cell line to TPT and SN-38 is mainly caused by reduced accumulation. The reduced accumulation appears to be mediated by a novel mechanism, probably related to impaired energy-dependent uptake of these topoisomerase drugs.

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confidence: 99%

Reduced cellular accumulation of topotecan: a novel mechanism of resistance in a human ovarian cancer cell line

Maliepaard²,

Nooter³

et al. 1998

“…In normal cells, topo I activity is probably required for gene transcription and possibly for DNA synthesis and replication (Slichenmyer et al, 1993). The topo I inhibitors are an exciting new class of antineoplastic agents with activity in patients with refractory solid tumour malignancies (reviewed in Potmesil, 1994).…”

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confidence: 99%

Variation in topoisomerase I gene copy number as a mechanism for intrinsic drug sensitivity

McLeod

Keith²

1996

Summary DNA topoisomerase I (topo I) is the principle target for camptothecin and its derivatives such as SN38. Levels of topo I expression vary widely between and within tumour types and the basis for this is poorly understood. We have used fluorescence in situ hybridisation to detect the topo I locus in a panel of breast and colon cancer cell lines. This approach has identified a range of topo I gene copies from 1 to 6 between the cell lines as a result of DNA amplification, polysomy and isochromosome formation. Topo I gene copy number was highly correlated with topo I expression, (r, = 0.92), and inversely correlated to sensitivity to a 1 h exposure to SN38 (r, = -0.904). This illustrates the significant impact of altered topo I gene copy number on intrinsic drug sensitivity and influences potential mechanisms for acquisition of drug resistance.

“…Recently several semisynthetic CPT analogues (Slichenmyer et al, 1993;Creemers et al, 1994;Potmesil, 1994) have been developed, aiming at reduced toxicity and sustained or improved activity.…”

Section: G1147211mentioning

confidence: 99%

The bioavailability of oral GI147211 (GG211), a new topoisomerase I inhibitor

Gerrits

Schellens²,

Creemers³

et al. 1997

Summary Topoisomerase I inhibitors are new compounds of interest for cancer chemotherapy. We performed a study with Gl147211, a new semisynthetic camptothecin analogue, to determine the absolute bioavailability of the drug given orally. Patients with a histologically confirmed diagnosis of a solid tumour refractory to standard forms of therapy were eligible for the study. Gi147211 was given orally on day 1 and as a 30-min infusion daily on days 2-5. The treatment course was repeated every 3 weeks. In subsequent patient cohorts, the dose of the oral formulation was escalated from 1.5 mg m-2 to 6.0 mg m-2; the dose for i.v. administration was fixed at 1.2 mg m-2. Plasma pharmacokinetics was performed on day 1 and 2 of the first course and on day 1 of the second course using a validated high-performance liquid chromatographic assay. Nineteen patients were entered into the study; one patient was not evaluable because the treatment course was stopped prematurely. Eighteen patients received a total of 47 treatment courses. The absolute bioavailability of G1147211 averaged 1.3 ± 5.2%. Drug appeared quickly in plasma with a median Tma, at 0.5 h. Fasting or fed state had no significant influence on the bioavailability of G1147211. The terminal half-life after administration of oral G1147211 was 6.85 ± 3.13 h, similar to the half-life after intravenous administration. The major toxicities were neutropenia and thrombocytopenia. Nadirs for neutropenia and thrombocytopenia occurred on day 8 and day 15 respectively. Other toxicities predominantly consisted of mild and infrequent nausea and vomiting, and fatigue. The oral administration of the drug is well tolerated. Oral administration of topoisomerase I inhibitor G1147211 results in a low bioavailability with relatively wide interpatient variation. The intravenous route of administration is advised for further development of this promising topoisomerase I inhibitor.