Pharmacokinetics of cyclophosphamide in man

Docetaxel, cyclophosphamide, and epirubicin: application of PBPK modeling to gain new insights for drug-drug interactions

The new adjuvant chemotherapy of docetaxel, epirubicin, and cyclophosphamide has been recommended for treating breast cancer. It is necessary to investigate the potential drug-drug Interactions (DDIs) since they have a narrow therapeutic window in which slight differences in exposure might result in significant differences in treatment efficacy and tolerability. To guide clinical rational drug use, this study aimed to evaluate the DDI potentials of docetaxel, cyclophosphamide, and epirubicin in cancer patients using physiologically based pharmacokinetic (PBPK) models. The GastroPlus™ was used to develop the PBPK models, which were refined and validated with observed data. The established PBPK models accurately described the pharmacokinetics (PKs) of three drugs in cancer patients, and the predicted-to-observed ratios of all the PK parameters met the acceptance criterion. The PBPK model predicted no significant changes in plasma concentrations of these drugs during co-administration, which was consistent with the observed clinical phenomenon. Besides, the verified PBPK models were then used to predict the effect of other Cytochrome P450 3A4 (CYP3A4) inhibitors/inducers on these drug exposures. In the DDI simulation, strong CYP3A4 modulators changed the exposure of three drugs by 0.71-1.61 fold. Therefore, patients receiving these drugs in combination with strong CYP3A4 inhibitors should be monitored regularly to prevent adverse reactions. Furthermore, co-administration of docetaxel, cyclophosphamide, or epirubicin with strong CYP3A4 inducers should be avoided. In conclusion, the PBPK models can be used to further investigate the DDI potential of each drug and to develop dosage recommendations for concurrent usage by additional perpetrators or victims.

Symptomatic hyponatremia induced by low-dose cyclophosphamide in patient with systemic lupus erythematosus: A case report

RATIONALE

Cyclophosphamide (CY) is an alkylating agent used widely to treat cancer and autoimmune diseases. Hyponatremia is a common adverse effect of high-dose and moderate-dose of intravenous CY, but is rare in patients treated with low-dose (<15 mg/kg).

PATIENT CONCERNS

A 52-year-old woman with new-onset systemic lupus erythematosus (SLE) was treated with low-dose cyclophosphamide (8 mg/kg, CY), but showed sudden headaches, disorientation and weakness. Laboratory examinations revealed severe isovolumic hyponatremia along with low-serum osmolality and high urine osmolality.

DIAGNOSIS

The acute hyponatremia was consistent with the syndrome of inappropriate antidiuretic hormone secretion (SIADH) and was an adverse event of low-dose CY, with no evidence of endocrine, cancer, pulmonary, or cerebral abnormalities relevant to the SIADH.

INTERVENTION

The hyponatremia was resolved after the supplementation of NaCl solution.

OUTCOMES

The hyponatremia was resolved without any complications.

LESSONS

Hyponatremia induced by low-dose CY should be recognized as an underlying life-threatening complication in clinical practice.

Hyponatraemia induced by low-dose intravenous pulse cyclophosphamide

BACKGROUND

Cyclophosphamide is an alkylating agent and was traditionally known to potentiate the renal action of vasopressin. Although low-dose intravenous pulse cyclophosphamide therapy is being used extensively in the treatment of malignant and rheumatological diseases, there have been only a few case reports of cyclophosphamide-induced hyponatraemia.

METHODS

Clinical data were retrospectively analysed from 84 patients (42 lupus nephritis; 42 non-Hodgkin's lymphoma; a total of 112 treatment episodes) admitted for intravenous pulse cyclophosphamide (500-750 mg/m(2)) therapy. In all patients, half-isotonic saline was used for prophylactic hydration. Cyclophosphamide-induced hyponatraemia was defined as serum sodium concentration <135 mEq/L at 24 hours after the therapy in patients whose basal serum sodium concentrations were normal.

RESULTS

After the low-dose intravenous pulse cyclophosphamide, serum sodium concentration significantly decreased from 139.9 +/- 3.5 to 137.9 +/- 5.1 mEq/L (P < 0.001). Cyclophosphamide-induced hyponatraemia occurred in 15 treatment episodes (13.4%) from 12 patients (14.3%). Patients with hyponatraemia were significantly older than those without hyponatraemia (57.3 +/- 14.7 vs. 40.0 +/- 17.0 years, P < 0.01). Hyponatraemia was associated with male sex on univariate analysis (P < 0.05), but not on multivariate analysis. No factors were found to independently predict the occurrence of cyclophosphamide-induced hyponatraemia when multivariate analysis was performed including parameters age, sex, underlying disease, presence or absence of comorbidities associated with hyponatraemia, presence or absence of concurrent medications associated with hyponatraemia and dose of cyclophosphamide.

CONCLUSIONS

Hyponatraemia occurring after low-dose intravenous pulse cyclophosphamide is not rare, especially when hypotonic solutions are adopted for hydration protocol. Thus, the use of hypotonic fluids should be avoided when using cyclophosphamide. Instead, isotonic solutions should be used if a forced diuresis is required.

Water intoxication following low-dose intravenous cyclophosphamide

Cyclophosphamide is frequently used for the treatment of severe lupus nephritis, but is very rarely associated with dilutional hyponatremia. Recently we experienced a case of water intoxication following low-dose intravenous cyclophosphamide. Five hours after one dose of intravenous pulse cyclophosphamide 750 mg, the patient developed nausea, vomiting, and general weakness. Serum sodium concentration revealed 114 mEq/L and her hyponatremia was initially treated with hypertonic saline infusion. Then her serum sodium concentration rapidly recovered to normal with water restriction alone. During the course of intravenous pulse cyclophosphamide therapy, one must be aware of the possibility of significant water retention.

Clinical pharmacokinetics of cyclophosphamide

Cyclophosphamide is an extensively used anticancer and immunosuppressive agent. It is a prodrug undergoing a complicated process of metabolic activation and inactivation. Technical difficulties in the accurate determination of the cyclophosphamide metabolites have long hampered the assessment of the clinical pharmacology of this drug. As these techniques are becoming increasingly available, adequate description of the pharmacokinetics of cyclophosphamide and its metabolites has become possible. There is incomplete understanding on the role of cyclophosphamide metabolites in the efficacy and toxicity of cyclophosphamide therapy. However, relationships between toxicity (cardiotoxicity, veno-occlusive disease) and exposure to cyclophosphamide and its metabolites have been established. Variations in the balance between metabolic activation and inactivation of cyclophosphamide owing to autoinduction, dose escalation, drug-drug interactions and individual differences have been reported, suggesting possibilities for optimisation of cyclophosphamide therapy. Knowledge of the pharmacokinetics of cyclophosphamide, and possibly monitoring the pharmacokinetics of cyclophosphamide in individuals, may be useful for improving its therapeutic index.

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.

Water intoxication induced by low-dose cyclophosphamide in two patients with systemic lupus erythematosus

Cyclophosphamide (CY) is an alkylating agent used to treat a variety of autoimmune disorders. Water intoxication is a well-known complication of high-dose intravenous (i.v.) CY, but is rare in patients treated with low dose i.v. CY. We describe two patients with lupus nephritis and water intoxication following low dose i.v. CY. The first patient was treated with oral prednisolone and azathioprine for eight weeks with inadequate response and persistent renal inflammatory activity. Eight hours after the first i.v. CY pulse she had a grand mal seizure. The second patient had WHO class III lupus nephritis, and after a single i.v. CY pulse developed vomiting, diarrhoea and grand mal seizures. They were both fluid-restricted and their serum sodium levels returned to normal. In conclusion, even at low doses i.v. CY may induce hyponatremia related to inappropriate antidiuretic hormone secretion. This potentially life-threatening complication of i.v. CY could be minimized by avoidance of overhydration following pulse i.v. CY.

Pharmaceuticals in the environment--a human risk?

Pharmaceuticals in the environment and their potential toxic effects are emerging research areas, which is also reflected in the drug approval regulation. This far, focus has mainly been directed toward potential effects on nature and wildlife. In this paper, human risk as a consequence of exposure via the environment has been addressed and assessed. The synthetic estrogen 17alpha-ethinylestradiol (EE2), the antibiotic phenoxymethylpenicillin (Pen V), and the antineoplastic drug cyclophosphamide (CP) were chosen as modeling substances based on criteria of receptor specificity, elevated risk for human population groups for which the pharmaceuticals are not therapeutically intended, different modes of action, and prescription frequency. Attention has been focused on emissions from the use phase and subsequent diffuse release via the sewer systems. A reasonable worst-case environmental fate and human exposure were estimated using the software EUSES on worst-case emission quantities. The results indicate a negligible human risk connected to the environmental exposure for these substances. Danish conditions have been used as the modeling area, but the results are assumed to be valid for regions with similar drug consumption profiles.

Simple relations between administered and internal doses in compartmental flow models

It is sometimes argued that the use of increasingly complex "biologically-based" risk assessment (BBRA) models to capture increasing mechanistic understanding of carcinogenic processes may run into a practical barrier that cannot be overcome in the near term: the need for unrealistically large amounts of data about pharmacokinetic and pharmacodynamic parameters. This paper shows that, for a class of dynamical models widely used in biologically-based risk assessments, it is unnecessary to estimate the values of the individual parameters. Instead, the input-output properties of such a model--specifically, the ratio of the area-under-curve (AUC) for any selected output to the AUC of the input--is determined by a single aggregate "reduced" constant, which can be estimated from measured input and output quantities. Uncertainties about the many individual parameter values of the model, and even uncertainties about its internal structure, are irrelevant for purposes of quantifying and extrapolating its input-output (e.g., dose-response) behavior. We prove that this is the case for the class of linear, constant-coefficient, globally stable compartmental flow systems used in many classical pharmacokinetic and low-dose PBPK models. Examples are cited that suggest that the value of the reduced parameter representing such a system's aggregate behavior may be relatively insensitive to changes in (and hence to uncertainties about) the values of individual parameters. The theory is illustrated with a model of pharmacokinetics and metabolism of cyclophosphamide (CP), a drug widely used in cancer chemotherapy and as an immunosuppressive agent.

Preclinical pharmacokinetics and stability of isophosphoramide mustard

Stability and preclinical pharmacokinetics of isophosphoramide mustard (IPM), an active metabolite of ifosphamide, were investigated using analytical methods developed in this laboratory. For stability evaluation of IPM we used a rapid, high-pressure liquid chromatographic (HPLC) method by which IPM is analyzed directly from aqueous solutions without derivatization on a 10-microns C-18 reversed-phase column with theophylline as the internal standard. IPM in sodium phosphate buffers was found to undergo pH-dependent first-order degradations. At pH 7.4 and 38 degrees C, the IPM solution showed a half-life of 45 min. A gas chromatographic-mass spectrometry (GC/MS) method for the analysis of IPM in plasma was also developed. This method utilized solid-phase extraction with deuterium-labeled IPM as the internal standard. The routine detection limit for the assay was 50 ng/ml with within-run and between-run coefficients of variation of 6% and 11%, respectively. By this method, stability of IPM in plasma and in RPMI 1640 tissue culture medium was evaluated, and its pharmacokinetics in the Sprague-Dawley rat following i.v. administration at 40 mg/kg were investigated. IPM was found to be more stable in these media, with half-lives in the range of 100 min. IPM plasma pharmacokinetics were found to decline monoexponentially with terminal half-lives ranging from 6.8 to 18.7 min and total clearance between 6.0 and 18.3 ml/min. Plasma protein binding of IPM was found to be 55%, and the partition ratio between plasma and red blood cells of 4.9 to 1, respectively. Cytotoxicity of IPM to L1210 cells was evaluated, and the results indicated that the IC50 with 1-h and 4-h exposure was 33 and 15 microM, respectively. Based on these data, IPM plasma levels in the rat declined below the IC50 in about 1 h at this dose. More frequent dosing or infusion may be necessary to maintain adequate drug levels for antitumor activity when IPM is administered directly.

Combined thin-layer chromatography-photography-densitometry for the quantitation of cyclophosphamide and its four principal urinary metabolites

A novel method for the quantitative determination of the anti-cancer drug cyclophosphamide and its principal urinary metabolites 4-oxocyclophosphamide, carboxyphosphamide, phosphoramide mustard and bis(2-chloroethyl)amine has been devised. The assay combines adsorption of drug-related material onto Amberlite XAD-2 and thin-layer chromatography with spot visualization using 4-(4-nitrobenzyl)pyridine, rapid photography and densitometry. The intra-assay coefficient of variation for each compound was less than 6%. The limit of detection of the assay was 1 microgram ml-1 for cyclophosphamide, phosphoramide mustard and bis(2-chloroethyl)amine and 0.5 microgram ml-1 for 4-oxocyclophosphamide and carboxyphosphamide. The method was validated for cyclophosphamide and 4-oxocyclophosphamide using gas chromatography. It is concluded that the method provides the first means of determining the full metabolic spectrum for cyclophosphamide in patients without recourse to the administration of radioisotopically labelled drug.

Induction of specific-locus and dominant-lethal mutations by cyclophosphamide and combined cyclophosphamide-radiation treatment in male mice

Cyclophosphamide is the most widely used antineoplastic agent. It is also used to condition patients for bone-marrow transplantations. Because of the general interest of this compound we initiated a systematic study of the induction of dominant-lethal and specific-locus mutations in male mice. In addition, we investigated the induction of specific-locus mutations by the combined treatment of cyclophosphamide and ionizing radiation. A dose of 40 mg/kg bw of cyclophosphamide caused dominant-lethal mutations in male mice only in the 1st and 2nd week after treatment. A dose of 120 mg/kg induced dominant-lethal mutations in the mating intervals 1-21 days posttreatment. No dominant lethal mutations were observed after the 3rd week. The same differential spermatogenic response was observed for the induction of specific-locus mutations. Cyclophosphamide induced recessive mutations exclusively in spermatozoa and spermatids. No mutations were recovered from treated spermatocytes and spermatogonia. In contrast to cyclophosphamide, radiation induces specific-locus mutations in all germ-cell stages. The pretreatment with cyclophosphamide 24 h before radiation enhanced the frequency of specific-locus mutations in spermatogonia. The distribution of the observed mutations among the 7 loci and their viability supports the hypothesis that these mutations were induced by radiation rather than by cyclophosphamide. The compound causes an immediate inhibition of DNA and RNA synthesis in spermatogonia. The inhibition very likely interferes with the repair process. The disturbance of the repair process is probably the cause of the synergistic effect for the induction of specific-locus mutations in spermatogonia of mice after pretreatment with cyclophosphamide 24 h before irradiation.

Monitoring the behaviour of 4-ketocyclophosphamide versus cyclophosphamide during capillary gas chromatography by mass spectrometry

Capillary Gas Chromatography (CGC) is capable of determining underivatized cyclophosphamide (CPA) using SCOT OV 275 columns. Then CPA is subjected to in situ degradation resulting in formation of a cyclization product which can be determined selectively in biological fluids. In routine bioanalysis however cyclization products of CPA metabolites might interfere, e.g. 4-keto CPA. In the present study possible formation of cyclization products of 4-keto CPA similar to CPA was monitored by Mass Spectrometry. Cyclization of 4-keto CPA in situ was demonstrated to occur, resulting in a product similar to that of CPA. Both cyclization products could be determined selectively and it appeared that in situ cyclization of 4-keto CPA was negligible (less than 5%), probably owing to extra stabilization of the CPA metabolite by keto-enol tautomerism as has been demonstrated by NMR.

Pharmacokinetics of anticancer drugs in children

Interpatient pharmacokinetic variability normally observed in adults is often of even greater magnitude in paediatric patients because of age-related maturation of physiological processes responsible for drug disposition. Several antineoplastic agents have shown age-related changes, including alterations in volume of distribution, hepatic (doxorubicin, cyclophosphamide), and renal (bleomycin, methotrexate) clearances. These differences in pharmacokinetics as a function of age alter systemic exposure to chemotherapy, and may alter the efficacy and toxicity profile for standard doses of antineoplastic drugs. The relationship of systemic exposure to toxicity has been most clearly defined for methotrexate. Clinical monitoring of methotrexate serum concentrations, and adjustment of folinic acid dosages and duration of rescue based on methotrexate disposition is now routine. More recently, pharmacodynamic data have been published for high-dose methotrexate, epipodophyllotoxins, cisplatin, and cytarabine (cytosine arabinoside), indicating a relation between drug disposition and toxicity or efficacy. Collectively, these data suggest that the pharmacokinetics of many anticancer drugs in children is different from adults, and that variability in drug disposition may have an important influence on toxicity or efficacy.

Phase I clinical and pharmacokinetic study of cyclophosphamide administered by five-day continuous intravenous infusion

A total of 14 patients, 7 male and 7 female, received in all 21 evaluable courses of cyclophosphamide administered by 5-day continuous infusion. Cyclophosphamide doses were escalated from 300 to 400 mg/m2 per day for 5 days and repeated every 21-28 days. The patient population had a median age of 55 years (range 38-76) and a median Karnofsky performance status of 80 (range 60-100). Only 1 patient had not received prior therapy; 5 patients had received only prior chemotherapy, 1 had received only prior radiotherapy, and 7 had received both. Tumor types were gastric (1), lung (2), colon (4), urethral adenocarcinoma (1), cervical (2), chondrosarcoma (1), melanoma (1), uterine leiomyosarcoma (1), and pancreatic (1). The dose-limiting toxicity was granulocytopenia, with median WBC nadir of 1700/microliter (range 100-4800) in 8 heavily pretreated patients treated at 350 mg/m2 per day for 5 days. One patient without heavy prior treatment received two courses at 400 mg/m2 and had WBC nadirs of 800/microliter and 600/microliter. WBC nadirs occurred between days 9 and 21 (median 14). Drug-induced thrombocytopenia occurred in only one patient (350 mg/m2 per day, nadir 85,000/microliter). Neither hyponatremia nor symptomatic hypo-osmolality was observed. Radiation-induced hemorrhagic cystitis may have been worsened in one patient. Nausea and vomiting were mild. Objective remissions were not observed. The maximum tolerated dose for previously treated patients is 350 mg/m2 per day for 5 days. This dose approximates the doses of cyclophosphamide commonly used with bolus administration. Plasma steady-state concentrations (Css) of cyclophosphamide, measured by gas liquid chromatography, were 2.09-6.79 micrograms/ml. Steady state was achieved in 14.5 +/- 5.9 h (mean +/- SD). After the infusion, cyclophosphamide disappeared from plasma monoexponentially, with a t 1/2 of 5.3 +/- 3.6 h. The area under the curve of plasma cyclophosphamide concentrations versus time (AUC) was 543 +/- 150 micrograms/ml h and reflected a cyclophosphamide total-body clearance (CLTB) of 103 +/- 31.6 ml/min. Plasma alkylating activity, assessed by p-nitrobenzyl-pyridine, remained steady at 1.6-4.3 micrograms/ml nor-nitrogen mustard equivalents. Urinary excretion of cyclophosphamide and alkylating activity accounted for 9.3% +/- 7.6% and 15.1% +/- 2.0% of the administered daily dose, respectively. The t1/2 and AUC of cyclophosphamide associated with the 5-day continuous infusion schedule are similar to those reported after administration of cyclophosphamide 1500 mg/m2 as an i.v. bolus.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacokinetics of "activated" cyclophosphamide and therapeutic efficacies

Estimation of "activated" cyclophosphamide (4-OH-CP) in blood of cancer patients and laboratory animals has revealed significant differences between pharmacokinetics of cyclophosphamide (CP) in man and laboratory animals after CP treatment. Whereas in blood of mice and rats relatively high concentrations of 4-OH-CP were found to exist for a relatively short time, in blood of humans only low, but longer-lasting, blood levels were detected after administration of comparable CP doses. In order to examine whether these different pharmacokinetic behaviors might account at least in part for the known differences of antitumor activity and toxicity of CP between humans and laboratory animals, the authors studied the influence of pharmacokinetics of activated CP on therapeutic efficacy and toxicity after injection of 4-(S-ethanol)-sulfido-cyclophosphamide (P1), a pro drug of activated CP, into nude mice bearing heterotransplanted human bladder sarcoma. With P1, which hydrolyzes quickly in blood to yield 4-OH-CP, different blood level shapes of 4-OH-CP could be established either by single bolus injection of P1 or by repetitive injection of a loading dose followed by several maintenance doses which caused nearly constant levels of activated CP for a longer time period. With these models it was found that 4-OH-CP showed more therapeutic efficacy when present in blood at relatively low levels for longer times than after bolus injection of the same dose resulting in a sharp peak level of activated CP. So after single intraperitoneal (IP) injection of 300 mg/kg P1 which caused a bioavailability of 36 mumol/ml-1/minute a 67% inhibition of tumor growth was achieved, whereas a tumor growth reduction of 83% was obtained after injection of the same dose in 6 fractions resulting in constant blood levels with a bioavailability of only 17 mumol/ml-1/minute. In contrast to the significant influence on antitumor efficacy of activated CP, practically no effect of pharmacokinetics on toxicity of 4-OH-CP could be observed. Therefore, the bioavailability of activated CP, which killed 50% of the animals, was determined to be approximately 89 mumol/ml-1/minute after adjustment of pharmacokinetics to yield constant levels and approximately 79 mumol/ml-1/minute after single bolus injection. The experiments presented show that by adjustment of pharmacokinetics the therapeutic index of P1, defined as bioavailability causing 50% of animals to die, referred to bioavailability causing 90% tumor growth inhibition, could be more than doubled.

The influence of myelosuppression on the response to chemotherapy in small cell bronchogenic carcinoma

In an attempt to clarify the relationship between myelosuppression and the response to chemotherapy, 127 cases of small cell lung cancer were reviewed. These patients received a total of four different drug combinations. The myelosuppression of the first chemotherapy course was reduced with the addition of one or two drugs to the basic Cytoxan (cyclophosphamide)-vincristine drug combination without a change in the incidence of remission. Patients with good performance status have less severe leukopenia and thrombocytopenia than those with poor performance status. Patients with complete and partial response have slightly more severe thrombocytopenia but not leukopenia than the nonresponders. It is concluded that the burden of tumor has a direct effect on the incidence of remission and myelosuppression suggesting that more severe toxicity is necessary to obtain remission in poor risk patients. Myelosuppression is required before assessing whether the patient has received an adequate amount of chemotherapeutic agent but is only a weak prognostic factor.

Comparative effects of ASTA Z 7557 (INN mafosfamide) and cyclophosphamide on hematopoiesis in mice

Acute effects of ASTA Z 7557 and Cyclophosphamide (Cy) on pluripotential (CFU-S), granulocytic (CFU-C), early erythroid (BFU-E) and late erythroid (CFU-E) progenitor cells in the bone marrow, as well as on RBC and WBC, were compared in F1 (CBA X C 57 BL) female mice. Dose-survival curves of both agents for CFU-S and CFU-C were found to be exponential, indicating that the effects of the drugs have no cell cycle dependency. At equimolar doses, marrow CFU-S and CFU-C contents appeared to decrease more rapidly with increasing doses of ASTA Z 7557 than with those of Cy. After a single dose of 200 mg/kg of each drug ( = 50% LD10), there was greater initial suppression of CFU-S, CFU-C, BFU-E and CFU-E in the Cy-treated animals than in ASTA Z 7557-treated mice. In both groups, however, the WBC had their nadir on Day 3 after treatment, followed by a return to normal by Day 8. In ASTA Z 7557-treated animals, the recovery of CFU-S, CFU-C and BFU-E was also completed by Day 8 after treatment. In Cy-treated mice, however, complete recovery of these cells was achieved on Day 15. Results indicate quantitative rather than qualitative differences between the marrow toxicities of ASTA Z 7557 and Cy in mice. Quantitative differences could be due to different pharmacokinetic properties of the agents, since Cy is excreted in partially unmetabolized form, and it might be that this inactive part of the agent grew increasing drug doses.

Pharmacokinetics of high-dose cyclophosphamide in patients with metastatic bronchogenic carcinoma

Cyclophosphamide (CP) was administered to eight patients with metastatic bronchial carcinoma in escalating doses of 1.5, 2.5, and 3.5 g/m2 at intervals of 3 weeks. The proportion of the administered dose converted into alkylating metabolites was similar for each dose and there was no evidence to suggest that the enzyme system responsible for activating CP was saturated even with the highest dose. Considerable between-patient variation in drug metabolism was observed, but within each patient the fraction metabolised remained constant.

Pharmacokinetics of cyclophosphamide in Kenyan Africans

The pharmacokinetics of cyclophosphamide was studied in 10 Kenyan Africans with Hodgkins lymphoma. The mean +/- s.d. elimination half-life (t1/2) was 7.5 +/- 1.38 h. The mean +/- s.d. volume of the central compartment (V1) was 0.35 +/- 0.12 l/kg and the apparent volume of distribution (V) was 0.64 +/- 0.06 l/kg. The microconstants k21, k12 and k10 were 1.81 +/- 0.84 h-1, 1.90 +/- 1.080 h-1 and 2.05 +/- 0.86 h-1 respectively (mean +/- s.d.).

Decreased plasma half-life of cyclophosphamide during repeated high-dose administration

Cyclophosphamide was given as IV doses of 50 mg/kg/day on each of four successive days as treatment for ovarian and lung cancer. Blood samples were taken at regular intervals and analysed for cyclophosphamide by gas liquid chromatography. The plasma half-lives (t 1/2) and volumes of distribution (V D) were calculated for each of the treatment days; t 1/2 was found to decrease with subsequent doses whereas V D was not significantly changed.

Clinical pharmacokinetics of commonly used anticancer drugs

The quantitative aspects of drug disposition in man of the commonly used antineoplastic agents, including cyclophosphamide, the nitrosoureas, cisplatin, methotrexate, cytarabine, 5-fluorouracil, doxorubicin, daunorubicin, bleomycin, vincristine, vinblastine, and vindesine are reviewed. Although the pharmacokinetic behaviour of these drugs has been adequately described in man, the chemical reactivity, the complexity of metabolism and disposition, the lack of simple, rapid and sensitive assays to measure plasma concentration, and the lack of defined therapeutic and toxic plasma concentrations have limited the application of routine drug monitoring in clinical oncology. With the exception of high dose methotrexate, drug doses and administration schedules remain empirical with a standard starting dose and subsequent dosage modifications determined by ensuing drug toxicities. However, many of the pharmacological characteristics of the drugs, such as their low therapeutic index, potentially life-threatening toxicities and wide individual variability in drug disposition, necessitate pharmacological monitoring. Comprehensive pharmacokinetic analysis of new and established antineoplastic agents does play a role in defining dosage, administration schedule, route of administration, and dosage modification in the presence of organ dysfunction. Consideration of the kinetics of these drugs in planning treatment regimens could lead to more rational, safer and possibly more efficacious use.

Mutagens in human urine: test with human peripheral lymphocytes

A test is described in which human peripheral blood is enclosed in dialysis bags and exposed to human urine. After the treatment, the blood is cultivated in the presence of bromodeoxyuridine, and metaphases are analyzed with respect to the frequencies of sister-chromatid exchanges (SCE). Urine from clinically healthy people and from epileptic patients on therapy with anti-epileptic drugs did not lead to an elevation of SCE frequencies. Urine from cancer patients treated with a multi-drug combination therapy including cyclophosphamide led to a significant increase in the frequencies of SCEs.

Biliary elimination of cyclophosphamide in man

A 72-year-old male with a lymphoma and obstructive jaundice received 900 mg cyclophosphamide IV as a part of a chemotherapeutic regimen whilst external biliary drainage was in progress. Plasma, urinary, and biliary pharmacokinetics of cyclophosphamide and nitrobenzylpyridine (NBP)-alkylating metabolites were studied. In 32 h 891 ml bile was collected, and this contained unchanged cyclophosphamide and NBP-alkylating material. Despite fluctuations in biliary flow, estimates of the half-life of cyclophosphamide from plasma, urine, and bile were similar. Good correlation existed between plasma and biliary cyclophosphamide concentrations after the initial plasma had been completed. The ratio of bile to plasma concentrations was 0.7 and showed no time dependence, as evidenced by a lack of hysteresis in the correlation curve. Of the administered dose, 3.5% was excreted as unchanged cyclophosphamide in the bile over 32 h. NBP-alkylating activity was found in bile up to 25 h but not after this time, despite the presence of unchanged cyclophosphamide in plasma. NBP-alkylating material was not found in the bile when it could not be detected in plasma, and vice versa.

Metabolism of high doses of cyclophosphamide

The excretion of cyclophosphamide and the enzymatically derived metabolites 4-ketocyclophosphamide and carboxyphosphamide has been measured in four patients after the administration of cyclophosphamide (5 g). At this dose the enzymes responsible for the biotransformation and detoxification of cyclophosphamide are not saturated. In two patients the metabolite profile was unaffected by a previous high dose of cyclophosphamide and in one patient a small primary dose did not alter metabolism.

Salivary elimination of cyclophosphamide in man

A total of 25 plasma and saliva samples were obtained from three hospital inpatients upon intravenous short-time infusion of cyclophosphamide; total doses infused ranged from 5.3 to 13.3 mg/kg. Despite interindividual variations, a correlation was found for plasma-to-saliva ratio of 1.61 +/- 0.53 (S.D.). Our Saliva-to-plasma ratio of 0.62 is in agreement with a recently reported ratio of 0.77. The ratio stayed constant throughout the observation period (0.5 to 8.4 hours), indicating that the noninvasive method of saliva sampling seems to be a useful approach in cyclophosphamide drug concentration monitoring in patients.

Cyclophosphamide and dimethylsulfoxide in the treatment of squamous carcinoma of the lung. Therapeutic efficacy, toxicity, and pharmacokinetics

To determine whether dimethylsulfoxide (DMSO) can potentiate antitumor activity of cyclophosphamide (CYC) in patients with squamous cell carcinoma of the lung, 14 patients were treated with 5 l of a 5% or 6% DMSO solution PO over 3 days and 1,500 mg CYC/m2 IV as a 60-min infusion on the third day of treatment. Serial blood, CSF, and urine samples were collected to assess the pharmacokinetics of CYC. Courses were repeated every 3-4 weeks. No antitumor responses were observed. Toxicity was mainly hematologic and similar to that of CYC alone. There was one death from infection during granulocytopenia. Nonhematologic toxicity was moderate to severe and included nausea (14 patients) and vomiting (five patients). The plasma pharmacokinetics of CYC in this study are similar to previously reported results for CYC alone, but the 24-h urinary excretion of CYC in our study is much lower than previously reported. Further studies in tumors more responsive to CYC may be warranted.

Two-compartment pharmacokinetic models: computer simulations of their characteristics and clinical consequences

Computer simulations of two-compartment models of drug disposition were made. These simulations show how variations in each of the three intercompartmental rate constants are related to altered blood and tissue concentrations. Various simulations following rapid injection or continuous infusion are presented to demonstrate the general behavior of such a pharmacokinetic model. A system of drug classification based on comparative values of a drug's intercompartmental rate constants is proposed, and characteristics and consequences of each class are discussed in terms of possible therapeutic results.

The pharmacokinetics of cyclophosphamide, phosphoramide mustard and nor-nitrogen mustard studied by gas chromatography in patients receiving cyclophosphamide therapy

1 Simple, accurate and specific gas-chromatographic methods for the estimation of derivatized phosphoramide and non-nitrogen mustards utilizing alkali-flame ionization detection are described.

2 The pharmacokinetics in plasma of cyclophosphamide, phosphoramide mustard, nor-nitrogen mustard and nitrobenzyl pyridine alkylating activity were investigated following administration of cyclophosphamide by intravenous and oral routes to patients with malignant disease

3 The mean T_½ for cyclophosphamide was 8.88 h (s.d. 1.25 h) and the apparent volume of distribution (V_dβ) was 0.74 l kg^-1 (s.d. 0.16 l kg^-1).

4 The decline in plasma concentration of phosphoramide mustard was biphasic, the longer T_½ being 8.68 h (s.d. 2.50 h). This was not significantly different from that of cyclophosphamide. This could indicate that the true biological T_½ for phosphoramide mustard is identical with or shorter than that of cyclophosphamide.

5 The plasma concentrations of phosphoramide mustard following cyclophosphamide doses of known therapeutic efficacy are probably insufficient to produce important cytotoxic effects. This suggests that if phosphoramide mustard is the major alkylating metabolite derived from cyclophosphamide, it is transported in the blood in precursor form.

6 The mean plasma T_½ of nor-nitrogen mustard was 3.31 h (s.d. 1.60 h) which was significantly different from that of cyclophosphamide.

7 The mean plasma T_½ of the nitrobenzylpyridine alkylating activity was 9.81 h (s.d. 4.18 h) and did not significantly differ from that of cyclophosphamide. Although the area under the plasma alkylating activity concentration, time curve is related to the T_½ of cyclophosphamide, the alkylating activity does not reflect the concentrations of the two plasma metabolites measured.

Some aspects of the gas--liquid chromatographic analysis of cyclophosphamide in plasma

The gas--liquid chromatography of cyclophosphamide has been extensively investigated. Several methods for the assay of cyclophosphamide in plasma are reported, those with the nitrogen--phosphorus-specific detection being most sensitive and selective.

[Blood level and urinary excretion of activated cyclophosphamide and its deactivation products in man (author's transl)]

Blood levels and urinary excretion of cyclophosphamide and its metabolites were determined in cancer patients receiving cyclophosphamide. Activated cyclophosphamide (4-hydroxycyclophosphamide aldophosphamide) was assayed by TLC after derivatisation to stable 4-(S-benzyl)-sulfido-cyclophosphamide. Twenty minutes after injection of 10(20) mg/kg cyclophosphamide mean peak levels of activated cyclophosphamide were found to be 1.4(2.6) nmol/ml. The rate constant for biotransformation (=activation) of cyclophosphamide in man (km = 0.132 h-1) was only 1/50 of the value found in the mouse whereas the elimination rate constant of activated cyclophosphamide (ke[M] approximately 6.78 h-1) was much higher equalling that of laboratory animals. 4-ketocyclophosphamide, carboxyphosphamide, and phosphoramidemustard reached their peak levels between 4 and 6 h after cyclophosphamide injection. Increasing quantities of cyclophosphamide metabolites were bound to plasma proteins reaching a constant level after 24 h lasted for several days. Fifty per cent of those metabolites were reversibly bound to plasma proteins. Within 24 h, the cumulative excretion of cyclophosphamide and its metabolites amounted to 50% of the dose applied. The main metabolites excreted were phosphoramide-mustard and carboxyphosphamide whereas only 2% consisted of activated cyclophosphamide. The significance of the different pharmacokinetics of cyclophosphamide in laboratory animals and man for the therapeutic index is discussed.

The disposition of cyclophosphamide in a group of myeloma patients

The disposition of cyclophosphamide and its alkylating metabolites was investigated in a group of myeloma patients with varying degrees of renal function impairment. No correlation between renal function and clearance of cyclophosphamide or its alkylating metabolites was found. No evidence of accumulation of cyclophosphamide or alkylating activity was found in four patients receiving radiolabelled cyclophosphamide. Renal function was found to be related to the reciprocal of the area under curve of alkylating activity, indicating that this area increased as renal function decreased. In view of the large nonrenal component of alkylating activity elimination and the large inter-subject variability, it is recommended that dose of cyclophosphamide is not altered in moderate impairment of renal function.

Pharmacokinetics of cyclophosphamide and alkylating activity in man after intravenous and oral administration

1 Plasma cyclophosphamide levels were estimated by gas-chromatography in seven patients following intravenous and oral cyclophosphamide administration. Plasma alkylating activity was determined by the nitrobenzyl pyridine (NBP) reaction.

2 The plasma T_½ after intravenous administration ranged from 5.97 to 12.37 h but after oral administration was shorter, 1.32-6.8 h. The mean total body clearance was 66.6 ml kg^-1 h^-1 after intravenous dosing and 93.1 ml kg^-1 h^-1 following oral dosing. V_dβ was 0.71 (0.10 s.d.) 1 kg^-1 suggesting that cyclophosphamide is distributed largely in body water.

3 The mean hepatic extraction ratio was 0.25, indicating a modest first pass metabolism. The metabolic clearance was 3.72 1 kg^-1 and the intrinsic hepatic clearance 5.17 1 kg^-1.

4 The mean renal clearance was 4.8 ml kg^-1 h^-1 with 6.5% of the administered dose excreted unchanged in the urine suggesting tubular reabsorption of cyclophosphamide.

5 The mean T_½ of plasma alkylating activity was 8.82 h, there being no significant difference following oral and intravenous administration. On average, 3.5 times the alkylating activity was produced by an oral dose of cyclophosphamide as compared to an intravenous dose. It is possible that this may reflect production of a different pattern of alkylating metabolites following cyclophosphamide administration by different routes. The clinical significance of these observations is unknown.

Pharmacokinetics of cyclophosphamide after prolonged low dose treatment in ovarian cancer patients

We have studied Cyclophosphamide (Cy) pharmacokinetics in 9 patients after their first intravenous dose of 100 mg, and in 9 patients after 6--13 months of continual treatment with the same dose every day. The half-life of Cy was shorter (p less than 0.05) in the latter group, the Co was greater (less than 0.01) and the Vd was clearly less (p less than 0.01). The AUC was unchanged.

Pharmacokinetics of intravenous cyclophosphamide in man, estimated by gas-liquid chromatography

A simple gas chromatographic assay utilising alkali flame ionisation detection is described for the estimation of cyclophosphamide as its trifluoroacetate derivative from plasma. Examination of five patients following intravenous cyclophosphamide gave values of 8.9 h (SD 2.7) for the half-life and 0.061 liters/h/kg (SD 0.011) for whole-body clearance of the drug.