Pharmacokinetics and metabolism of beta-2'-deoxythioguanosine and 6-thioguanine in man

Mechanisms of telomere maintenance and associated therapeutic vulnerabilities in malignant gliomas

A majority of cancers (~85%) activate the enzyme telomerase to maintain telomere length over multiple rounds of cellular division. Telomerase-negative cancers activate a distinct, telomerase-independent mechanism of telomere maintenance termed alternative lengthening of telomeres (ALT). ALT uses homologous recombination to maintain telomere length and exhibits features of break-induced DNA replication. In malignant gliomas, the activation of either telomerase or ALT is nearly ubiquitous in pediatric and adult tumors, and the frequency with which these distinct telomere maintenance mechanisms (TMMs) is activated varies according to genetically defined glioma subtypes. In this review, we summarize the current state of the field of TMMs and their relevance to glioma biology and therapy. We review the genetic alterations and molecular mechanisms leading to telomerase activation or ALT induction in pediatric and adult gliomas. With this background, we review emerging evidence on strategies for targeting TMMs for glioma therapy. Finally, we comment on critical gaps and issues for moving the field forward to translate our improved understanding of glioma telomere maintenance into better therapeutic strategies for patients.

Mass Balance Studies, with a Focus on Anticancer Drugs

The importance of in-depth knowledge about the pharmacokinetics of a drug is evident because pharmacokinetic behaviour may correlate with activity and toxicity. The most elaborate pharmacokinetic investigation is a so-called mass balance study employing a radioactive tracer. A mass balance study investigates the plasma pharmacokinetics and excretion of both the unchanged drug and the total radioactivity (drug and metabolites), and allows elucidation of the metabolic fate of a drug. The main objective is the maximum recovery of the radioactive dose in urine and faeces. However, there is little concrete guidance on how to perform or assess such a mass balance study. Based on a critical review of the available literature and regulation, we discuss the design, conduct and evaluation of mass balance studies. The study must be preceded by pre-clinical studies to optimise the design. Compliance with available regulation (US FDA is more specific than European guidelines), choice of radioisotope (tritium may be lost and is consequently used less often), selection of subjects and safe administration of a relevant dose are critical. The drug (both the labelled and the non-labelled portion of the dose) must be prepared, characterised and administered, and the biological samples must be collected and analysed properly to obtain reliable and meaningful results. A recent development is the use of highly sensitive accelerator mass spectrometry for (14)C-detection, which allows the use of much lower amounts of radioactivity. Calculating the mass balance requires quantification of any possible losses of radioactivity during the study, e.g. during administration (adsorption to tubing). Total recovery should be at least 90% of the administered dose. A lower recovery, which is not uncommon, must be explained by biological factors such as a long decay half-life, irreversible binding to tissue components, or loss through expiration. The extent and relevance of metabolism is determined by comparing the pharmacokinetics of the unchanged drug and the total radioactivity. Metabolic profiling supplements existing knowledge of metabolism. We illustrate the important aspects of a mass balance study with literature on anticancer drugs. This review could serve as guidance for future mass balance studies.

Pharmacokinetic studies in children with cancer

We reviewed the current status of our knowledge of pharmacokinetics and pharmacodynamics of some anti-neoplastic drugs, used in the treatment of childhood cancer. Extrapolation of data from pharmacokinetic studies in adults to the paediatric population is often not feasible. Specific studies in children are needed. Of all reviewed anti-neoplastic drugs methotrexate appears to be most extensively studied. Methotrexate pharmacokinetics is correlated with toxicity and response to therapy, and it has been shown that individualized adaptive dosing of methotrexate is correlated with a better response to therapy without increasing toxicity in children with ALL and osteosarcoma. Of most of the other reviewed anti-neoplastic drugs it is demonstrated that pharmacokinetics is correlated with toxicity, and of some drugs a relationship of pharmacokinetics with response to therapy is demonstrated as well. In case of cytarabine, etoposide, and teniposide, individualized dosing also appears to be feasible. However, there is no evidence that this strategy improves response to therapy. Specifically data on pharmacokinetic and pharmacodynamic correlations and effect of pharmacokinetically guided, individualized dosing are important for the design of optimal cancer chemotherapy for individual patients. Unfortunately for a considerable number of anti-neoplastic drugs these specific data are lacking in children and future research is needed.

Is 6-thioguanine more appropriate than 6-mercaptopurine for children with acute lymphoblastic leukaemia?

The cytotoxic activity of 6-mercaptopurine (6-MP) is affected by thiopurine methyltransferase (TPMT), a genetically regulated and variable intracellular enzyme. 6-Thioguanine (6-TG), a closely related thiopurine, is less affected by that enzyme and so it may be a more reliable drug-at least for patients with constitutionally high TPMT activity. We attempted to assess its suitability as an alternative by comparing the pharmacokinetics of both drugs in a small group of children with lymphoblastic leukaemia (ALL). Patients were included who were in their second or subsequent remission, who would otherwise have received 6-MP, and on whom pharmacokinetic data concerning 6-MP metabolism had been collected in a previous remission. Plasma 6-TG concentrations were assayed following an oral dose of 40 mg m-2, and the accumulation and fluctuation of intracellular (erythrocyte, RBC) 6-TG nucleotides (6-TGNs) were measured at regular intervals during daily oral therapy. Seven children were studied. Plasma 6-TG concentrations were low and cleared within 6 h of oral dosing. At 7 days, 6-TGN concentrations ranged from 959 to 2361 pmol 8 x 10(-8) RBCs, in all cases significantly higher (P = 0.002) than those produced by the same patients on 6-MP. After a total therapy time of 35 patient months, a modest rise of alanine aminotransferase was seen on one occasion, otherwise no toxicity apart from myelosuppression was encountered. In the context used, 6-TG appears well tolerated and produces higher concentrations of intracellular cytotoxic metabolites than 6-MP. For children constitutionally 'resistant' to the traditional drug, if not all, it may be a preferable alternative.

Radiosensitization of two murine fibrosarcomas with 6-thioguanine

In Vivo murine tumor experiments were carried out to determine whether 6-thioguanine (6-TG) could enhance the cytotoxic effects of radiation on tumors. The combined effects of single and fractionated x-irradiation were evaluated on the transplanted methylcholanthrene induced fibrosarcoma (Meth-A) in BALB/c mice, a moderately radioresponsive tumor and on the radiation induced fibrosarcoma (RIF) in C3H/He mice, a highly radioresistant tumor. The combined treatment of single administration of 6-TG (25 mg/kg) and of x-irradiation (20 Gy) on Meth-A tumors produced more than 90% tumor control, whereas the radiation alone resulted in less than 5% tumor control. The radiosensitizing effect by 6-TG was higher when the drug was administered either 1 to 8 hr prior to or 24 hr after x-irradiation. The dose modification factor of single dose 6-TG (10 mg/kg) is estimated to be 1.47 for Meth-A tumor and 1.25 for RIF tumor. The tumor control rates of fractionated irradiation alone and with concomitant 6-TG in Meth-A tumors were 14% and 59%, respectively. Based on the studies reported here and well documented pharmacokinetics in humans, it is suggested that combined radiation therapy and 6-TG may provide an enhanced therapeutic effect even in tumor varieties where the drug has no apparent anti-tumor activity on non-irradiated cells.

6-Thioguanine: high-dose 2-H infusions in goats

6-Thioguanine (6TG) is poorly absorbed after oral administration. Bolus injections of 6TG result in high peak concentrations with relatively short-lived plasma concentrations. In vitro studies have shown the importance of prolonged exposure to 6TG. Therefore we administered 6TG by infusion at a dose rate of 2 mg/h over 2 h. In three goats we determined the plasma concentration-time curves of 6TG and its riboside (6TGR). A steady state was reached for 6TG and was almost reached for 6TGR within the 2 h of infusion. In one experiment we obtained several samples of CSF and observed good penetration of 6TG and 6TGR into CSF. Urinary excretion of 6TG and 6TGR was also quantitated. The amount of drug and metabolite excreted later than 4 h after the end of the infusion was negligible. By infusing 6TG, the problems of both erratic absorption after oral administration and acute renal toxicity after bolus injection, can be averted. In our opinion prolonged infusions of 6TG may be of advantage in humans suffering from actively proliferating malignant diseases, and thus should be studied.

Potentiation of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU)-induced cytotoxicity in 9L cells by pretreatment with 6-thioguanine

9L Rat brain tumor cells were treated with 0.2 microM 6-thioguanine for 48 hr, which produced a 40% cell kill, a small (15%) inhibition of cell growth, and an accumulation of cells in S-phase. Maximum incorporation of [14C]6-thioguanine into cellular DNA occurred after 24 hr of incubation; 70% of the label was incorporated into DNA as 6-thio-2'-deoxyguanosine. Pretreatment of 9L cells for 48 hr with 0.2 microM 6-thioguanine potentiated the cytotoxicity of 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) by 50% with a dose enhancement ratio of 1.5, and caused a 30% increase in the number of BCNU-induced sister chromatid exchanges (SCEs) and a 50% increase in DNA crosslinks formed, compared to treatment with BCNU alone. Used as a single agent, 6-thioguanine induced a significant number of SCEs. Results suggest that these effects may be related to the increased formation of DNA crosslinks, possibly as the result of the formation of S6-(2-chloroethyl)-6-thioguanine in cellular DNA.