Evaluation of the use of static and dynamic models to predict drug-drug interaction and its associated variability: impact on drug discovery and early development.

Simcyp, a population-based simulator, is widely used for evaluating drug-drug interaction (DDI) risks in healthy and disease populations. We compare the prediction performance of Simcyp with that of mechanistic static models using different types of inhibitor concentrations, with the aim of understanding their strengths/weaknesses and recommending the optimal use of tools in drug discovery/early development. The inclusion of an additional term in static equations to consider the contribution of hepatic first pass to DDIs (AUCR(hfp)) has also been examined. A second objective was to assess Simcyp's estimation of variability associated with DDIs. The data set used for the analysis comprises 19 clinical interactions from 11 proprietary compounds. Except for gut interaction parameters, all other input data were identical for Simcyp and static models. Static equations using an unbound average steady-state systemic inhibitor concentration (I(sys)) and a fixed fraction of gut extraction and neglecting gut extraction in the case of induction interactions performed better than Simcyp (84% compared with 58% of the interactions predicted within 2-fold). Differences in the prediction outcomes between the static and dynamic models are attributable to differences in first-pass contribution to DDI. The inclusion of AUCR(hfp) in static equations leads to systematic overprediction of interaction, suggesting a limited role for hepatic first pass in determining inhibition-based DDIs for our data set. Our analysis supports the use of static models when elimination routes of the victim compound and the role of gut extraction for the victim and/or inhibitor in humans are not well defined. A fixed variability of 40% of predicted mean area under the concentration-time curve ratio is recommended.

The dihydroorotase inhibitor 5-aminoorotic acid inhibits the metabolism in the rat of the cardioprotective drug dexrazoxane and its one-ring open metabolites.

Dexrazoxane (ICRF-187) is clinically used as a doxorubicin cardioprotective agent and to prevent anthracycline extravasation injury. It may act by preventing iron-based oxygen free radical damage through the iron-chelating ability of its metabolite N,N'-[(1S)-1-methyl-1,2-ethanediyl]bis[(N-(2-amino-2-oxoethyl)]glycine (ADR-925). Dexrazoxane undergoes an initial metabolism to its two one-ring open intermediates [N-(2-amino-2-oxoethyl)-N-[(1S)-2-(3,5-dioxo-1-piperazinyl)-1-methylethyl]glycine (B) and N-(2-amino-2-oxoethyl)-N-[(2S)-2-(3,5-dioxo-1-piperazinyl)propyl]glycine (C)] and is then further metabolized to its presumably active metal-chelating form ADR-925. We previously showed that the first ring opening reaction is catalyzed by dihydropyrimidinase and the second by dihydroorotase (DHOase), but not vice versa. To determine whether DHOase was important in the metabolism of dexrazoxane, its metabolism and that of B and C to ADR-925 were measured in rats that were pretreated with the DHOase inhibitor 5-aminoorotic acid. In rats pretreated with 5-aminoorotic acid the area-under-the-curve concentration of ADR-925 was reduced 5.3-fold. In rats treated with a mixture of B and C, the maximum concentration of ADR-925 in the plasma was significantly decreased in rats pretreated with 5-aminoorotic acid, which indicates that DHOase directly metabolized B and C. Both heart and liver tissue levels of ADR-925 in rats were also greatly reduced by pretreatment with 5-aminoorotic acid. Together these results indicate that the metabolism of dexrazoxane and of B and C is mediated by DHOase. These results provide a mechanistic basis for the antioxidant cardioprotective activity of dexrazoxane.

Metabolism of the one-ring open metabolites of the cardioprotective drug dexrazoxane to its active metal-chelating form in the rat.

Dexrazoxane (ICRF-187) is clinically used as a doxorubicin cardioprotective agent and may act by preventing iron-based oxygen free radical damage through the iron-chelating ability of its fully hydrolyzed metabolite ADR-925 (N,N'-[(1S)-1-methyl-1,2-ethanediyl]-bis[(N-(2-amino-2-oxoethyl)]glycine). Dexrazoxane undergoes initial metabolism to its two one-ring open intermediates and is then further metabolized to its active metal ion-binding form ADR-925. The metabolism of these intermediates to the ring-opened metal-chelating product ADR-925 has been determined in a rat model to identify the mechanism by which dexrazoxane is activated. The plasma concentrations of both intermediates rapidly decreased after their i.v. administration to rats. A maximum concentration of ADR-925 was detected 2 min after i.v. bolus administration, indicating that these intermediates were both rapidly metabolized in vivo to ADR-925. The kinetics of the initial appearance of ADR-925 was consistent with formation rate-limited metabolism of the intermediates. After administration of dexrazoxane or its two intermediates, ADR-925 was detected in significant levels in both heart and liver tissue but was undetectable in brain tissue. The rapid rate of metabolism of the intermediates was consistent with their hydrolysis by tissue dihydroorotase. The rapid appearance of ADR-925 in plasma may make ADR-925 available to be taken up by heart tissue and bind free iron. These studies showed that the two one-ring open metabolites of dexrazoxane were rapidly metabolized in the rat to ADR-925, and thus, these results provide a mechanism by which dexrazoxane is activated to its active metal-binding form.

Metabolism of the cardioprotective drug dexrazoxane and one of its metabolites by isolated rat myocytes, hepatocytes, and blood.

The metabolism of the antioxidant cardioprotective agent dexrazoxane (ICRF-187) and one of its one-ring open metabolites to its active metal ion binding form N,N'-[(1S)-1-methyl-1,2-ethanediyl-]bis[(N-(2-amino-2-oxoethyl)]glycine (ADR-925) has been investigated in neonatal rat myocyte and adult rat hepatocyte suspensions, and in human and rat blood and plasma with a view to characterizing their hydrolysis-activation. Dexrazoxane is clinically used to reduce the iron-based oxygen free radical-mediated cardiotoxicity of the anticancer drug doxorubicin. Dexrazoxane may act through its hydrolysis product ADR-925 by removing iron from the iron-doxorubicin complex, or binding free iron, thus preventing oxygen radical formation. Our results indicate that dexrazoxane underwent partial uptake and/or hydrolysis by myocytes. A one-ring open metabolite of dexrazoxane underwent nearly complete dihydroorotase-catalyzed metabolism in a myocyte suspension. Hepatocytes that contain both dihydropyrimidinase and dihydroorotase completely hydrolyzed dexrazoxane to ADR-925 and released it into the extracellular medium. Thus, in hepatocytes, the two liver enzymes acted in concert, and sequentially, on dexrazoxane, first to produce the two ring-opened metabolites, and then to produce the metabolite ADR-925. We also showed that the hydrolysis of one of these metabolites was promoted by Ca2+ and Mg2+ in plasma, and thus, further metabolism of these intermediates likely occurs in the plasma after they are released from the liver and kidney. In conclusion, these studies provide a nearly complete description of the metabolism of dexrazoxane by myocytes and hepatocytes to its presumably active form, ADR-925.

Pharmacokinetics of etoposide in cancer patients treated with high-dose etoposide and with dexrazoxane (ICRF-187) as a rescue agent.

PURPOSE: The pharmacokinetics of etoposide were studied in cancer patients with brain metastases treated with high-dose etoposide in order to determine if the pharmacokinetics were altered by the use of dexrazoxane as a rescue agent to reduce the extracerebral toxicity of etoposide. METHODS: Etoposide plasma levels were determined by HPLC. RESULTS: The etoposide pharmacokinetics described by a monophasic first-order elimination model were found to be similar to other reported data in other settings and at similar doses. CONCLUSIONS: The pharmacokinetics of etoposide were unaffected by dexrazoxane rescue.

The metabolites of the cardioprotective drug dexrazoxane do not protect myocytes from doxorubicin-induced cytotoxicity.

The clinically approved cardioprotective agent dexrazoxane (ICRF-187) and two of its hydrolyzed metabolites (a one-ring open form of dexrazoxane and ADR-925) were examined for their ability to protect neonatal rat cardiac myocytes from doxorubicin-induced damage. Dexrazoxane may protect against doxorubicin-induced damage to myocytes through its strongly metal-chelating hydrolysis product ADR-925, which could act by displacing iron bound to doxorubicin or chelating free or loosely bound iron, thus preventing site-specific iron-based oxygen radical damage. The results of this study showed that whereas dexrazoxane was able to protect myocytes from doxorubicin-induced lactate dehydrogenase release, neither of the metabolites displayed any protective ability. Dexrazoxane also reduced apoptosis in doxorubicin-treated myocytes. The ability of dexrazoxane and its three metabolites to displace iron from a fluorescence-quenched trapped intracellular iron-calcein complex was also determined to see whether the metabolites were taken up by myocytes. Although ADR-925 was taken up in the absence of calcium in the medium, in the presence of calcium, its uptake was greatly slowed, presumably because it formed a complex with calcium. Both of the one-ring open metabolites were taken up by myocytes and displaced iron from its complex with calcein. These results suggest either that the anionic metabolites do not have the same access to iron pools in critical cellular compartments, that their uptake is slowed in the presence of calcium, or, less likely, that dexrazoxane protects by some other mechanism.

Metabolism of dexrazoxane (ICRF-187) used as a rescue agent in cancer patients treated with high-dose etoposide.

PURPOSE: The study was undertaken to determine the metabolism of dexrazoxane (ICRF-187) to its one-ring open hydrolysis products and its two-rings opened metal-chelating product ADR-925 in cancer patients with brain metastases treated with high-dose etoposide. In this phase I/II trial dexrazoxane was used as a rescue agent to reduce the extracerebral toxicity of etoposide. METHODS: Dexrazoxane and its one-ring open hydrolysis products were determined by HPLC and ADR-925 was determined by a fluorescence flow injection assay. RESULTS: The two one-ring open hydrolysis intermediates of dexrazoxane appeared in the plasma at low levels upon completion of dexrazoxane infusion and then rapidly decreased with half-lives of 0.6 and 2.5 h. A plasma concentration of 10 micro M ADR-925 was also detected at the completion of the dexrazoxane i.v. infusion period, indicating that dexrazoxane was rapidly metabolized in vivo. A plateau level of 30 micro M ADR-925 was maintained for 4 h and then slowly decreased. The pharmacokinetics of dexrazoxane were found to be similar to other reported data in other settings and at lower doses. CONCLUSIONS: The rapid appearance of ADR-925 in plasma may make ADR-925 available to be taken up by heart tissue and bind free iron. These results suggest that the dexrazoxane intermediates are enzymatically metabolized to ADR-925 and provide a pharmacodynamic basis for the antioxidant cardioprotective activity of dexrazoxane.

Dihydroorotase catalyzes the ring opening of the hydrolysis intermediates of the cardioprotective drug dexrazoxane (ICRF-187).

The enzyme kinetics of the hydrolysis of the one-ring open metabolites of the antioxidant cardioprotective agent dexrazoxane [ICRF-187; (+)-1,2-bis(3,5-dioxopiperazin-1-yl)propane] to its active metal ion binding form ADR-925 [N,N'-[(1S)-1-methyl-1,2-ethanediyl]bis[N-(2-amino-2-oxoethyl)glycine] by dihydroorotase (DHOase) has been investigated by high-performance liquid chromatography (HPLC). A spectrophotometric detection HPLC assay for dihydroorotate was also developed. Dexrazoxane is clinically used to reduce the iron-based oxygen free radical-mediated cardiotoxicity of the anticancer drug doxorubicin. DHOase was found to catalyze the ring opening of the metabolites with an apparent V(max) that was 11- and 27-fold greater than its natural substrate dihydroorotate. However, the apparent K(m) for the metabolites was 240- and 550-fold larger than for dihydroorotate. This report is the first that DHOase might be involved in the metabolism of a drug. Furosemide inhibited DHOase, but the neutral 4-chlorobenzenesulfonamide did not. Because dihydroorotate, the one-ring open metabolites, and furosemide all have a carboxylate group, it was concluded that a negative charge on the substrate strengthened binding to the positively charged active site. The presence of DHOase in the heart may explain the cardioprotective effect of dexrazoxane. Thus, dihydropyrimidinase and DHOase acting in succession on dexrazoxane and its metabolites to form ADR-925 provide a mechanism by which dexrazoxane is activated to exert its cardioprotective effects. The ADR-925 thus formed may either remove iron from the iron-doxorubicin complex, or bind free iron, thus preventing oxygen radical formation.

The doxorubicin-cardioprotective drug dexrazoxane undergoes metabolism in the rat to its metal ion-chelating form ADR-925.

PURPOSE: Dexrazoxane is clinically used as a doxorubicin-cardioprotective agent and may act by preventing iron-based oxygen free-radical damage through the iron-chelating ability of ADR-925. The metabolism of dexrazoxane (ICRF-187) to its one-ring open hydrolysis products and its rings-opened metal-chelating product ADR-925 was determined in a rat model in order to identify the mechanism by which dexrazoxane acts. METHODS: A new fluorescence detection flow injection assay utilizing the metal-chelating dye calcein was developed to detect ADR-925 in blood plasma. Dexrazoxane and its one-ring open metabolites were determined by HPLC. RESULTS: ADR-925 was detected within 5 min of i.v. administration of dexrazoxane to rats, suggesting that dexrazoxane is rapidly metabolized in vivo. The plasma concentrations of ADR-925 exceeded those of both one-ring open intermediates at 30 min and those of dexrazoxane by 80 min and reached a maximum at 80 min, and then slowly decreased. CONCLUSIONS: The rapid appearance of ADR-925 in plasma may make ADR-925 available to be taken up by heart tissue and bind free iron. These results indicate that the one-ring open dexrazoxane intermediates are enzymatically metabolized to ADR-925 and provide a pharmacodynamic basis for the antioxidant cardioprotective activity of dexrazoxane.