Population pharmacokinetics of cyclophosphamide and its metabolites 4-hydroxycyclophosphamide, 2-dechloroethylcyclophosphamide, and phosphoramide mustard in a high-dose combination with Thiotepa and Carboplatin

The anticancer prodrug cyclophosphamide (CP) is activated by the formation of 4-hydroxycyclophosphamide (4OHCP), which decomposes into phosphoramide mustard (PM). This activation pathway is inhibited by thiotepa. CP is inactivated by formation of 2-dechloroethylcyclophosphamide (2DCECP). The aim of this study was to develop a population pharmacokinetic model describing the complex pharmacokinetics of CP, 4OHCP, 2DCECP, and PM when CP is administered in a high-dose combination with thiotepa and carboplatin. Patients received a combination of CP (1000-1500 mg/m/d), carboplatin (265-400 mg/m/d), and thiotepa (80-120 mg/m/d) administered in short infusions over 4 days. Twenty blood samples were collected per patient per course. Concentrations of CP, 4OHCP, 2DCECP, PM, thiotepa, and tepa were determined in plasma. Using NONMEM, an integrated population pharmacokinetic model was used to describe the pharmacokinetics of CP, 4OHCP, 2DCECP, and PM, including the already described processes of autoinduction of CP and the interaction with thiotepa. Data were available on 35 patients (70 courses). The pharmacokinetics of CP were described with a 2-compartment model, and those of 4OHCP, 2DCECP, and PM with 1-compartment models. Before onset of autoinduction, it was assumed that CP is eliminated through a noninducible pathway accounting for 20% of total CP clearance, whereas 2 inducible pathways resulted in formation of 4OHCP (75%) and 2DCECP (5%). It was assumed that 4OHCP was fully converted to PM. Induction of CP metabolism was mediated by 2 hypothetical amounts of enzyme whose quantities increased in time in the presence of CP (kenz=0.0223 and 0.0198 hours). Induction resulted in an increased formation of 4OHCP (approximately 50%), PM (approximately 50%), and 2DCECP (approximately 35%) during the 4-day course, and concomitant decreased exposure to CP (approximately 50%). The formation of 2DCECP was not inhibited by thiotepa. Apparent volumes of distribution of CP, PM, and 2DCECP could be estimated being 43.7, 55.5, and 18.5 L, respectively. Exposure to metabolites varied up to 9-fold. The complex population pharmacokinetics of CP, 4OHCP, 2DCECP, and PM in combination with thiotepa and carboplatin has been established and may form the basis for further treatment optimization with this combination.

Aprepitant inhibits cyclophosphamide bioactivation and thiotepa metabolism

BACKGROUND

Patients receiving the highly emetogenic high-dose chemotherapy regimen with cyclophosphamide, thiotepa and carboplatin (CTC) may benefit from the neurokin-1 receptor antagonist aprepitant in addition to standard anti-emetic therapy. As aprepitant has been shown to be a moderate inhibitor of the cytochrome P450 (CYP) 3A4 isoenzyme, its effect on the pharmacokinetics and metabolism of cyclophosphamide and thiotepa was evaluated. Moreover, preliminary results on the clinical efficacy of aprepitant in the CTC regimen are reported.

PATIENTS AND METHODS

Six patients were enrolled in a protocol that employed a 4-day course of CTC high-dose chemotherapy with cyclophosphamide (1,500 mg/m2/day), thiotepa (120 mg/m2/day) and carboplatin (AUC 5 mg min/ml/day). Two patients received the tCTC protocol, which comprises two-third of the dose of CTC. In addition to standard anti-emetic therapy, the patients received aprepitant from one day before the start of their course until 3 days after chemotherapy. Blood samples were collected on days one and three of the course and analyzed for cyclophosphamide and its activated metabolite 4-hydroxycyclophosphamide, thiotepa and its main active metabolite tepa. The influence of aprepitant on the pharmacokinetics of cyclophosphamide and thiotepa was analyzed using a population pharmacokinetic analysis including a reference population of 49 patients receiving the same chemotherapy regimen without aprepitant and sampled under the same conditions. The frequency of nausea and vomiting in the six patients receiving CTC was compared with those of the last 22 consecutive patients receiving CTC chemotherapy without aprepitant. Inhibitory activity of aprepitant on cyclophosphamide and thiotepa metabolism was also tested in human liver microsomes.

RESULTS

In our patient population, the rate of autoinduction of cyclophosphamide (P=0.040) and the formation clearance of tepa (P<0.001) were reduced with 23% and 33% when aprepitant was co-administered, respectively. Exposures to the active metabolite 4-hydroxycyclophosphamide and tepa were therefore reduced (5% and 20%, respectively) in the presence of aprepitant. In human liver microsomes, the 50% inhibitory concentrations (IC50) of aprepitant for inhibition of cyclophosphamide (IC50=1.3 microg/ml) and thiotepa (IC50=0.27 microg/ml) metabolism were within the therapeutic range. Patients receiving aprepitant experienced less frequently CINV both during and after the CTC course compared with the reference population (nausea 3.7 days vs. 5.8 days, P=0.052; vomiting 0.5 days vs. 4.8 days, P<0.001).

CONCLUSION

Aprepitant inhibited both cyclophosphamide and thiotepa metabolism, most probably due to inhibition of the CYP 3A4 and/or 2B6 isoenzymes. The effects of this interaction are, however, small compared to the total variability. Addition of aprepitant may provide superior protection against vomiting in patients receiving the highly emetogenic high-dose CTC chemotherapy.

Effects of co-medicated drugs on cyclophosphamide bioactivation in human liver microsomes

The alkylating agent cyclophosphamide (CP) is a prodrug requiring cytochrome P-450-mediated bioactivation to form the active 4-hydroxycyclophosphamide (4OHCP). Modifications in the rate of CP bioactivation may have implications for the effectiveness of CP therapy, especially in high-dose regimens. In this study, agents frequently co-administered with CP in high-dose chemotherapy regimens were tested for their possible inhibition of the bioactivation of CP in human liver microsomes. The Km and Vmax values for the conversion of CP to 4OHCP were 93 microM and 4.3 nmol/h.mg, respectively. No inhibition was observed for aciclovir, carboplatin, ciprofloxacine, granisetron, mesna, metoclopramide, ranitidine, roxitromycin and temazepam. Inhibition was observed for amphotericin B, dexamethasone, fluconazole, itraconazole, lorazepam, ondansetron and thiotepa, with IC50 values of 50, >100, >50, 5, 15, >100 and 1.25 microM, respectively. For all but thiotepa, these IC50 values were higher than the therapeutic drug levels and thus considered of no clinical relevance. We conclude that of the tested co-medicated agents, only thiotepa inhibited metabolism of CP to 4OHCP at clinically relevant concentrations, and may thereby influence therapeutic and toxic responses of CP therapy.

Significant induction of cyclophosphamide and thiotepa metabolism by phenytoin

PATIENT AND METHOD

A 42-year-old male patient with relapsing germ-cell cancer was enrolled in a salvage protocol that employed two 4-day courses of CTC high-dose chemotherapy with cyclophosphamide (1,500 mg m(-2) day(-1)), thiotepa (120 mg m(-2) day(-1)), and carboplatin, followed by peripheral blood progenitor cell support. From five days before the start of the second CTC course the patient received phenytoin for generalized epileptic seizures. Blood samples were collected on day 1 of both CTC courses and analyzed for cyclophosphamide and its activated metabolite 4-hydroxycyclophosphamide, and for thiotepa and its main active metabolite tepa.

RESULTS

Exposure (expressed as area under the plasma concentration vs time curve) to 4-hydroxycyclophosphamide and tepa in the second CTC course was increased by 51% and 115%, respectively, compared with the first CTC course, whereas exposure to cyclophosphamide and thiotepa was significantly reduced (67% and 29%, respectively). Because high exposure to 4-hydroxycyclophosphamide and tepa correlates with increased toxicity, the treatment risk of this patient was significantly increased. Therefore doses were reduced on the third day of the second course.

CONCLUSION

It was concluded that phenytoin significantly induces both cyclophosphamide and thiotepa metabolism, most probably by induction of the cytochrome p450 enzyme system. This potential clinical significant interaction should be taken into account when phenytoin is administered in combination with cyclophosphamide and thiotepa in clinical practice.

Accuracy, Feasibility, and Clinical Impact of Prospective Bayesian Pharmacokinetically Guided Dosing of Cyclophosphamide, Thiotepa, and Carboplatin in High-Dose Chemotherapy

Purpose: Relationships between toxicity and pharmacokinetics have been shown for cyclophosphamide, thiotepa, and carboplatin (CTC) in high-dose chemotherapy. We prospectively evaluated whether variability in exposure to CTC and their activated metabolites can be decreased with pharmacokinetically guided dose administration and evaluated its clinical effect.

Experimental Design: Patients received multiple 4-day courses of cyclophosphamide (1,000–1,500 mg/m2/d), thiotepa (80–120 mg/m2/d), and carbop latin (area under the plasma concentration-time curve 3.3–5 mg × min/mL/d). Doses were adapted on day 3 based on pharmacokinetic analyses of cyclophosphamide, 4-hydroxycyclophosphamide, thiotepa, tepa, and carboplatin done on day 1 using a Bayesian algorithm. Doses were also adjusted before and during second and third courses. Observed toxicity was compared with that in patients receiving standard dose CTC (n = 43).

Results: A total of 46 patients (108 courses) were included. For cyclophosphamide, thiotepa, and carboplatin, a total of 39, 58, and 65 dose adaptations were done within courses and 17, 40, and 43 before courses. The precision within which the target exposure was reached improved compared with no adaptation, especially after within-course adaptations (precision for cyclophosphamide, thiotepa, and carboplatin is 19%, 16%, and 13%, respectively); &gt;85% led to an exposure within ±25% of the target compared with 60% without dose adjustments. Toxicity was similar to that in a reference population, although the incidence of veno-occlusive disease was reduced.

Conclusions: Bayesian pharmacokinetically guided dosing for CTC was feasible and led to a marked reduction in variability of exposure.

Accuracy, feasibility, and clinical impact of prospective Bayesian pharmacokinetically guided dosing of cyclophosphamide, thiotepa, and carboplatin in high-dose chemotherapy

PURPOSE

Relationships between toxicity and pharmacokinetics have been shown for cyclophosphamide, thiotepa, and carboplatin (CTC) in high-dose chemotherapy. We prospectively evaluated whether variability in exposure to CTC and their activated metabolites can be decreased with pharmacokinetically guided dose administration and evaluated its clinical effect.

EXPERIMENTAL DESIGN

Patients received multiple 4-day courses of cyclophosphamide (1,000-1,500 mg/m2/d), thiotepa (80-120 mg/m2/d), and carbop latin (area under the plasma concentration-time curve 3.3-5 mg x min/mL/d). Doses were adapted on day 3 based on pharmacokinetic analyses of cyclophosphamide, 4-hydroxycyclophosphamide, thiotepa, tepa, and carboplatin done on day 1 using a Bayesian algorithm. Doses were also adjusted before and during second and third courses. Observed toxicity was compared with that in patients receiving standard dose CTC (n = 43).

RESULTS

A total of 46 patients (108 courses) were included. For cyclophosphamide, thiotepa, and carboplatin, a total of 39, 58, and 65 dose adaptations were done within courses and 17, 40, and 43 before courses. The precision within which the target exposure was reached improved compared with no adaptation, especially after within-course adaptations (precision for cyclophosphamide, thiotepa, and carboplatin is 19%, 16%, and 13%, respectively); >85% led to an exposure within +/-25% of the target compared with 60% without dose adjustments. Toxicity was similar to that in a reference population, although the incidence of veno-occlusive disease was reduced.

CONCLUSIONS

Bayesian pharmacokinetically guided dosing for CTC was feasible and led to a marked reduction in variability of exposure.

Simultaneous quantification of cyclophosphamide, 4-hydroxycyclophosphamide, N,N',N"-triethylenethiophosphoramide (thiotepa) and N,N',N"-triethylenephosphoramide (tepa) in human plasma by high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry

The alkylating agents cyclophosphamide (CP) and N, N', N"-triethylenethiophosphoramide (thiotepa) are often co-administered in high-dose chemotherapy regimens. Since these regimens can be complicated by the occurrence of severe and sometimes life-threatening toxicities, pharmacokinetically guided administration of these compounds, to reduce variability in exposure, may lead to improved tolerability. For rapid dose adaptations during a chemotherapy course, we have developed and validated an assay, using liquid chromatography coupled with electrospray tandem mass spectrometry (LC/MS/MS), for the routine quantification of CP, thiotepa and their respective active metabolites 4-hydroxycyclophosphamide (4OHCP) and N, N', N"-triethylenephosphoramide (tepa) in plasma. Because of the instability of 4OHCP in plasma, the compound is derivatized with semicarbazide (SCZ) immediately after sample collection and quantified as 4OHCP-SCZ. Sample pretreatment consisted of protein precipitation with a mixture of methanol and acetronitrile using 100 microl of plasma. Chromatographic separation was performed on an Zorbax Extend C18 column (150 x 2.1 mm i.d., particle size 5 microm), with a quick gradient using 1 mM ammonia solution and acetonitrile, at a flow-rate of 0.4 ml min(-1). The analytical run time was 10 min. The triple quadrupole mass spectrometer was operating in the positive ion mode and multiple reaction monitoring was used for drug quantification. The method was validated over the concentration ranges 200-40,000 ng ml(-1) for CP, 50-5000 ng ml(-1) for 4OHCP-SCZ and 5-2500 ng ml(-1) for thiotepa and tepa, using 100 microl of human plasma. These dynamic concentration ranges proved to be relevant in daily practice. Hexamethylphosphoramide was used as an internal standard. The coefficients of variation were <12% for both intra-day and inter-day precisions for each compound. Mean accuracies were also between the designated limits (+/- 15%). This robust and rapid LC/MS/MS assay is now successfully applied for routine therapeutic drug monitoring of CP, thiotepa and their metabolites in our hospital.

Sorption of thiotepa to polyurethane catheter causes falsely elevated plasma levels

Central venous access catheters are commonly used in clinical oncology. The double lumen variant is applied in pharmacokinetic studies for simultaneous administration and blood sampling when frequent blood collections are necessary. Occlusion of one lumen, a common complication, necessitates the investigator perform blood sampling through the administration lumen after interrupting the infusion. Plasma concentrations measured in this sample can be influenced by sorption of the previously infused compound to the catheter lumen. In this study, the quality of cyclophosphamide, thiotepa, and carboplatin plasma concentrations is investigated when sampling is performed through the administration lumen.