Pharmacokinetics and pharmacodynamics of carboplatin administered in a high-dose combination regimen with thiotepa, cyclophosphamide and peripheral stem cell support

Ototoxicity associated with high-dose carboplatin for patients with previously treated germ cell tumors

BACKGROUND

High-dose carboplatin is an essential part of curative high-dose chemotherapy (HDCT) for patients with previously treated germ cell tumors (GCTs). Although hearing loss (HL) is a known side effect of HDCT, data on its severity and characteristics are limited.

METHODS

Eligible patients received HDCT for GCTs from 1993 to 2017 and had audiograms before and after HDCT. HL severity was classified by American Speech-Language-Hearing Association criteria, and mean change in hearing threshold at each frequency (0.25-8 kHz) was estimated from pre- to post-HDCT and between HDCT cycles.

RESULTS

Of 115 patients (median age, 32 years), 102 (89%) received three cycles of HDCT. Of 106 patients with normal hearing to mild HL in the speech frequencies (0.5-4 kHz) before HDCT, 70 (66%) developed moderate to profound HL in the speech frequencies after HDCT. Twenty-five patients (22%) were recommended for hearing aids after HDCT. Patients with moderate to profound HL isolated to the higher frequencies (6-8 kHz) before HDCT were more likely to develop moderate to profound HL in the speech frequencies after HDCT (94% vs. 61%; p = .01) and to be recommended for hearing aids (39% vs. 18%; p = .05).

CONCLUSIONS

HL was frequent after HDCT for GCTs, with most patients developing at least moderate HL in the speech frequencies and approximately one in five recommended for hearing aids. Moderate to profound HL isolated to high frequencies at baseline was predictive of more clinically significant hearing impairment after HDCT.

PLAIN LANGUAGE SUMMARY

Some patients with germ cell tumors, the most common malignancy in adolescent and young adult men, are not cured with standard-dose chemotherapy and require high-dose chemotherapy (HDCT). Using detailed hearing assessments of patients receiving HDCT, we found that most patients developed significant hearing loss and that one in five needed hearing aids. Thus, strategies to reduce this side effect are urgently needed, and all patients receiving HDCT should have a hearing test after therapy.

Accurate determination of glomerular filtration rate in adults for carboplatin dosing: Moving beyond Cockcroft and Gault

OBJECTIVE

Carboplatin is a cytotoxic chemotherapy drug developed in the 1980s which is still widely used today across various tumour types. Despite its common application, there remains a significant controversy and practice variation on its unique method of dosing by area under the curve (AUC). One potential reason for this variability stems from the reliance of using an estimated glomerular filtration rate (eGFR) as an extrapolation of the measured GFR (mGFR) which the commonly used Calvert equation was originally validated for. This review takes a novel and collaborative nephro-oncology approach to highlight the historical evolution of carboplatin dosing, methods for estimating GFR and its relative performance in the application of carboplatin dosing for adult patients.

DATA SOURCES

We reviewed all pertinent publications comparing carboplatin AUC-based dosing in adult patients based on the various methods of GFR measurements or estimations in order to provide a comprehensive description of each method's advantages and risks.

DATA SUMMARY AND CONCLUSIONS

The Cockcroft-Gault equation has been widely studied but newer eGFR equations, such as the CKD-Epidemiology Collaboration (CKD-EPI) or Janowitz-Williams equation have outperformed the Cockcroft-Gault in recent studies.

Diversity of dose-individualization and therapeutic drug monitoring practices of platinum compounds: a review

Introduction: Platinum-derived drugs are commonly used for the treatment of solid tumors. The differences in chemical structures of these molecules lead to different pharmacological properties, in terms of indication, efficacy, and toxicity. Their pharmacokinetics (PK) differ according to their respective renal elimination and have led to many studies investigating their dose optimization. Area covered: This review attempts to summarize and compare PK and pharmacodynamics of cisplatin, carboplatin, and oxaliplatin, with an emphasis on differences of dose calculations and opportunities for therapeutic drug monitoring (TDM) in various patient populations. Expert opinion: Although cisplatin and carboplatin can be considered as analogs since they share the same DNA interacting properties, the slower hydrolysis of the latter results in a better safety profile. Carboplatin is the only drug in oncology to be administrated according to a target area under the curve of concentration versus time, considering that its PK variability is almost fully explained by renal function, not by body size. This enables individual dosing based on predicted carboplatin clearance (along with patients renal characteristics) or on actual clearance with TDM, especially in a high-dose protocol.

Pharmacokinetics of high-dose carboplatin in children undergoing high-dose chemotherapy and autologous stem cell transplantation with BSA-based dosing

Body surface area (BSA)-based carboplatin dosing is used in various centers due to practical issues of renal function-based dosing with area under the curve (AUC) measurement. Pharmacokinetic (PK) analysis of high-dose carboplatin was performed in pediatric solid tumor patients undergoing high-dose chemotherapy (HDCT) and autologous stem cell transplantation (ASCT) with BSA-based dosing to calculate the AUCs achieved with this dosing method and to find the correlation between the PK and the renal functions and the adverse events. Carboplatin was administered as once daily intravenous doses at 300, 400, or 500 mg/m²/day over 1 h for 3 or 4 days. On the first and the last day of carboplatin administration, PK samplings were done at 0, 1, 2, and 5 h and only at 0 h on any other days. Mean AUC on the first and the last day were 4.85 ± 0.95 min × mg/mL and 5.27 ± 1.04 min × mg/mL, respectively (n = 23). Overall, negative correlations between the renal functions and the AUCs were mild to moderate, but they were stronger in nephrectomized patients. ⁵¹Cr-EDTA clearance decreased with statistical significance with each additional dose of carboplatin (P = 0.020). Optimal high-dose carboplatin dosing method and optimal target AUCs for the different tumors need further analysis.

Dose-banding of carboplatin: rationale and proposed banding scheme

Background. In dose-banding (DB) prescribed doses of cancer chemotherapy are fitted to doseranges or ‘bands’ and standard doses for each band are provided using a selection of pre-filled infusions or syringes, either singly or in combination. DB is used for several drugs where dose is based on body surface area. No DB-scheme has been reported for carboplatin, which, in clinical practice, is routinely dosed according to renal function.

Study objective. To assess the rationale for DB of carboplatin with regards to factors that influence dosing accuracy, develop a DB scheme, and discuss its potential use and limitations.

Methods. Prospective evaluations of carboplatin area under the plasma concentration – time curve (AUC) following application of the Calvert-formula were identified by a literature search. A relevant carboplatin dose range for construction of a DB-scheme with Calvert-formula based doses was obtained from published glomerular filtration rate distributions for patients receiving carboplatin.

Results. A DB-scheme was developed for individually calculated carboplatin doses of 358–1232 mg, with 35 mg increments between each standard dose and a maximum deviation of 4.7% from prescribed dose. The proposed DB-scheme covers the GFR-ranges 47–221 mL/min and 26–151 mL/min for patients receiving doses based on the target AUCs of 5 and 7 mg/mL/min, respectively.

Conclusion. There is a strong scientific rationale to support DB of carboplatin. The proposed banding scheme could introduce benefits to patients and healthcare staff but, as with other DB schemes, should be validated with prospective clinical and pharmacokinetic studies to confirm safety and efficacy.

Is routine audiometric evaluation necessary in gynaecologic tumour patients undergoing chemotherapy?

BACKGROUND

Our objective was to assess the auditory function of gynaecological tumour patients who had received cytotoxic agents and to determine their associated risk of ototoxicity.

PATIENTS AND METHODS

87 patients who had undergone chemotherapy for gynaecological malignancies were investigated. Of these patients, 79% had breast cancer, and 14% ovarian cancer. All of the patients had a subjective assessment of their hearing function on a visual analogue scale. Audiometric tests were performed before and at 9 weeks, 18 weeks and 3 months after completion of chemotherapy.

RESULTS

The age of the patients ranged from 32 to 71 years (mean age of 53.5 ± 10.5 years). The average subjective rating of the patients' hearing function was 83.0 ± 17.2 before and 84.8 ± 16.9 3 months after completion of chemotherapy. No significant audiometric change at either the speech hearing frequency range (0.5-2 KHz) or high frequencies was observed in the patients after chemotherapy. There was also no significant difference in the hearing threshold of the patients who had received platinum analogue-based chemotherapy compared to non-platinum analogue-based chemotherapy.

CONCLUSION

Hearing loss is uncommon in patients treated with the typical gynaecological chemotherapy protocols. Hence, routine audiometric testing in these patients is not necessary.

Chemosensitization of endometrial cancer cells through AKT inhibition involves FOXO1

OBJECTIVE

Endometrial cancer is the most common type of gynecologic cancer in the United States. In this study, we propose that inhibition of the AKT pathway sensitizes cells to chemotherapeutic agents by increasing FOXO1 expression.

METHODS

Ishikawa and RL95 cells were treated with the AKT inhibitor (API-59CJ-OMe) alone and in combination with carboplatin or paclitaxel. Cells were counted using a hemocytometer and cell cycle analysis done with flow cytometry. Apoptosis was measured with TUNEL and Annexin V/DAPI staining. FOXO1 protein expression and localization was done using immunofluorescent staining of cells. Finally, the adenovirus containing triple mutant FOXO1 was used to overexpress the constitutively active FOXO1 in Ishikawa cells and its effects on cell viability were studied.

RESULTS

Treatment with 6 microM API-59CJ-OME resulted in preferential cell death in Ishikawa and RL95 cells compared to another endometrial cancer cell line, ECC1 after 48 h of treatment. API-59CJ-OME treatment of Ishikawa cells resulted in cell cycle arrest in the G2/M phase. The addition of API-59CJ-OME to carboplatin resulted in a synergistic increase in cell death by apoptosis compared to the responses to each agent separately. Treatment with API-59CJ-OME, carboplatin, paclitaxel or the combinations for 24 h increased nuclear expression of FOXO1 in Ishikawa cells. Overexpression of FOXO1 caused 37% of the cells to die within 24 h. Addition of carboplatin to the AD-FOXO1 expressing cells further increased cell death to 71%.

CONCLUSIONS

Inhibition of AKT signaling potentiates cell death in Ishikawa and RL95 cells when combined with carboplatin through mechanisms involving FOXO1 activation.

Chemotherapy Dosing Part II: Alternative Approaches and Future Prospects

This overview follows on from part I, which described the current practices used in chemotherapy dosing and the paucity of scientific evidence to support them. In part II, alternative approaches are discussed, both in terms of scientific rationale and practical application. These include therapeutic drug monitoring, the use of pharmacokinetic-pharmacodynamic relationships, flat-fixed dosing, Bayesian modelling and dose banding.

Flat Dosing of Carboplatin Is Justified in Adult Patients with Normal Renal Function

PURPOSE

The Calvert formula is a widely applied algorithm for the a priori dosing of carboplatin based on patients glomerular filtration rate (GFR) as accurately measured using the 51Cr-EDTA clearance. Substitution of the GFR in this formula by an estimate of creatinine clearance or GFR as calculated by formulae using serum creatinine (SCR; Cockcroft-Gault, Jelliffe, and Wright) is, however, routine clinical practice in many hospitals. The goal of this study was to validate this practice retrospectively in a large heterogeneous adult patient population.

EXPERIMENTAL DESIGN

Concentration-time data of ultrafilterable platinum of 178 patients (280 courses, 3,119 samples) with different types of cancer receiving carboplatin-based chemotherapy in conventional and high doses were available. Data were described with a linear two-compartment population pharmacokinetic model. Relations between SCR-based formulae for estimating renal function and carboplatin clearance were investigated.

RESULTS

None of the tested SCR-based estimates of renal function were relevantly related to the pharmacokinetic variables of carboplatin. Neither SCR (median, 51; range, 18-124 micromol/L) nor the estimated GFR using the three different formulae was related to carboplatin clearance.

CONCLUSIONS

Our data do not support the application of modifications of the Calvert formula by estimating GFR from SCR in the a priori dosing of carboplatin in patients with relatively normal renal function (creatinine clearance, >50 mL/min). For targeted carboplatin exposures, the original Calvert formula, measuring GFR using the 51Cr-EDTA clearance, remains the method of choice. Alternatively, in patients with normal renal function, a flat dose based on the mean population carboplatin clearance should be administered.

Understanding drug ototoxicity: molecular insights for prevention and clinical management

Ototoxicity is a trait shared by aminoglycoside and macrolide antibiotics, loop diuretics, platinum-based chemotherapeutic agents, some NSAIDs and antimalarial medications. Because their benefits in combating certain life-threatening diseases often outweigh the risks, the use of these ototoxic drugs cannot simply be avoided. In this review, the authors discuss some of the most frequently used ototoxic drugs and what is currently known about the cell and molecular mechanisms underlying their noxious effects. The authors also provide suggestions for the clinical management of ototoxic medications, including ototoxic detection and drug monitoring. Understanding the mechanisms of drug ototoxicity may lead to new strategies for preventing and curing drug-induced hearing loss, as well as developing new pharmacological drugs with less toxic side effects.

Relationship Between Cisplatin Administration and the Development of Ototoxicity

Purpose

To determine the auditory toxicity associated with dose- and schedule- intensive cisplatin/gemcitabine chemotherapy in non–small-cell lung carcinoma patients.

Patients and Methods

Patients were treated with gemcitabine followed by cisplatin according to an interpatient dose-escalation scheme. Patients were randomly assigned to receive treatment once a week for 6 weeks or once every 2 weeks for 4 weeks. The following cohorts of patients were treated with a reversed schedule once every 2 weeks, in which cisplatin was followed by gemcitabine. The dose-intensity of cisplatin was equal in both schedules. Audiometric evaluations were obtained for each ear at several frequencies. Mean hearing loss after cisplatin treatment was computed for each dose level at each tested frequency in each ear at baseline and subsequent follow-up audiometry. Pure tone averages (PTAs) were also calculated. The pharmacokinetics of cisplatin was determined to study the correlation among the maximum drug concentration, the area under the curve of unbound platinum, and the development of ototoxicity.

Results

A total of 328 audiograms were analyzed. At the higher frequencies, a more severe hearing impairment was recorded. Most patients showed a decrease in hearing thresholds at dosages above 60 mg/m2 cisplatin at the higher frequencies. PTAs at 1, 2, and 4 kHz show a mean hearing loss of 19 dB after cisplatin administration at dosages above 90 mg/m2. Threshold shifts at 8 and 12.5 kHz after cisplatin administration were experienced at dosages above 60 mg/m2.

Conclusion

Hearing loss after cisplatin therapy occurs mainly at high frequencies and at cisplatin dosages more than 60 mg/m2. It is more pronounced when cisplatin is given once every 2 weeks.

Sparse Sampling Design for Therapeutic Drug Monitoring of Sequentially Administered Cyclophosphamide, Thiotepa, and Carboplatin (CTC)

The alkylating agents cyclophosphamide, thiotepa, and carboplatin (CTC) are administered simultaneously in high-dose chemotherapy regimens. This regimen is sometimes complicated by severe organ toxicities, which may be caused by interindividual variability in the pharmacokinetics of the agents. Monitoring plasma levels and adapting doses may reduce variability in exposure to the compounds and their metabolites. The aim of this study was to develop and validate a sparse sampling design for routine dose individualization of cyclophosphamide, thiotepa, and carboplatin both during and between courses in the CTC regimen. Models describing the population pharmacokinetics of the prodrug cyclophosphamide (4000 or 6000 mg/m) and its activated metabolite 4-hydroxycylophosphamide, thiotepa (320 or 480 mg/m), and its equipotent metabolite tepa, and carboplatin (1067 or 1600 mg/m) in the 4-day CTC regimen have been developed previously using the program NONMEM. Based on these models, plasma concentrations were calculated in 20 groups of 50 simulated patients in each group during multiple courses of therapy, and the exposure, expressed as area under the plasma concentration-versus-time curve (AUC), was calculated. Subsequently, individual model-predicted AUCs were calculated for all courses, based on selected simulated plasma concentrations during the first course of therapy. Strategies were compared by assessment of their predictive performance of the AUC and their applicability in clinical practice. Withdrawal of 3 samples on the first day of the course at 190, 290, and 400 minutes after start of cyclophosphamide infusion resulted in unbiased and precise first course AUC predictions of 4-hydroxycylophosphamide, thiotepa and tepa, and carboplatin (precision [root mean squared relative prediction error, %RMSE] 20%, 16%, 8.8%, respectively). Applying this same strategy at day 3 (or 4) of the course, with an additional sample at 600 minutes on both days, resulted in unbiased and precise predictions of the AUC of a following course (%RMSE 21%, 18%, 17%, respectively). Prospective validation of the strategies in 23 additional patients yielded comparable results. It can be concluded that a good and useful sparse sampling design was developed for precise and accurate estimation of the AUCs of 4-hydroxycyclophosphamide, thiotepa and tepa, and carboplatin in the CTC regimen. This method is valuable in pharmacokinetically guided dose adaptation both during and between CTC courses.

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); >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.

Pharmacokinetics of high-dose chemotherapy

There is considerable variation in the severity of preparative regimen-related toxicity (RRT) in hematopoietic stem-cell transplantation (HSCT). This variation has been recognized to be due, in part, to the wide variation in the pharmacokinetics (PK) of high-dose chemotherapy (HDC). Consequently, therapeutic drug modeling and pharmacokinetic-directed therapy (PKDT) represents an attractive strategy in this setting. Advances in our understanding of drug metabolism, the nature of the active metabolites, and the ability to measure drug concentrations have led to the point where for some agents it is now possible to treat to a given PK end point with a great deal of reliability. In-depth knowledge of the PK and pharmacodynamics (PD) associations of the agents employed in the high-dose setting will make possible more efficient research into preparative regimen dosing intensity and comparisons of different preparative regimens as well as safer HSCT overall. In this review, we discuss PK and PD studies of high-dose cyclosphamide, melphalan, thiotepa, carmustine, cisplatin, carboplatin, paclitaxel, docetaxel, and busulfan.

Individualised dosing strategy for high-dose carboplatin in patients with germ cell cancer

In contrast to conventional chemotherapy, carboplatin is still dosed per unit of body surface area (BSA) in high-dose chemotherapy protocols in clinical practice. To individualise dosing, a population pharmacokinetic model for poor-risk germ cell tumour patients receiving 1500 mg m(-2) carboplatin was developed. The typical central volume of distribution (19.9 l) and typical clearance (110 ml min(-1)) corresponded approximately to the extracellular fluid space or glomerular filtration rate, respectively. The covariate analysis identified several patient-specific factors. Carboplatin clearance was significantly related to creatinine clearance and body height, explaining 73% of the interindividual variability. Thus, an equation to predict individual clearance prior to treatment was developed (CL=0.41 x creatinine clearance+1.05 x body height-124.4). The relative frequency of developing toxicity increased significantly with higher AUC values for different types of toxicity. In addition, overall nonhaematological toxicity correlated significantly with exposure of carboplatin, leading to the assessment of a target AUC. Based on the prediction of individual clearance and the definition of a target AUC associated with moderate toxicity, an individualised dosing equation is proposed. Retrospectively, the individualised dosing strategy would have led to a higher dose on average and a broader range to be administered, compared to empirical dosing per unit BSA in the high-dose setting.

Toxicity of high-dose carboplatin: ultrafiltered and not total plasma pharmacokinetics is of clinical relevance

Combination chemotherapy containing high-dose carboplatin is a therapeutic option for patients with poor-risk germ cell tumors. Since such treatment is relatively toxic, pharmacokinetic characteristics of total and ultrafiltered platinum, representing different species of platinum complexes formed, were investigated in 29 patients in relation to toxicity. The goodness-of-fit parameters revealed that a two-compartment model with weighting (1/C2i) resulted in the best fit. The terminal half-life of total platinum > 5 days was fourfold longer than that of ultrafiltered platinum, indicating long-term retention. At stem cell rescue, an ultrafiltered concentration of < or = 0.223 microg/ml did not impair hematological recovery. Ultrafiltered but not total platinum concentrations or AUC values were significantly correlated to an increase in creatinine concentration and different types of toxicity associated with high-dose treatment. Since the fraction unbound is time dependent and highly variable among patients, measuring ultrafiltered concentrations is highly recommended as they represent the clinically relevant platinum complexes in high-dose treatment.

High-dose carboplatin and regimen-related toxicity following autologous bone marrow transplant

Pharmacokinetic analysis of carboplatin dosing suggests a more accurate prediction of toxicity when the dose is based on the area under the plasma concentration vs time curve (AUC) instead of body surface area (BSA). We retrospectively calculated the carboplatin AUC of 117 patients who received an autologous stem cell transplant following a conditioning regimen consisting of carboplatin 1800 mg/m(2) and cyclophosphamide 6000 mg/m(2) to identify whether higher carboplatin exposure resulted in an increase in regimen-related non-hematologic toxicities. The most common non-hematologic toxicities were gastrointestinal and hepatic. Twenty (17%) patients experienced additional > or =grade 2 toxicity, specifically, renal toxicity significantly associated with a higher median AUC of 10.2 mg/ml(-1) min (P = 0.001). Prior platinum therapy was also significantly associated with toxicity (P = 0.052). While carboplatin dose based on BSA varied minimally (median 990 (range 450-1340) mg, the calculated AUC showed a near four-fold range of exposure (median 7.8 (range 3.6 to 13.8) mg/ml(-1) min). These data suggest a relationship between non-hematologic adverse events and the estimated AUC. Prospective trials will be necessary to identify the target carboplatin AUC which optimizes outcome and minimizes toxicity in the autologous transplant setting.

Relationship between exposure and toxicity in high-dose chemotherapy with cyclophosphamide, thiotepa and carboplatin

BACKGROUND

High-dose chemotherapy in combination with peripheral blood progenitor cell transplantation is widely used in the treatment of several malignancies. The use of high-dose chemotherapy can be complicated by the occurrence of severe and sometimes life threatening toxicity. A wide interpatient variability in toxicity is encountered, which may be caused by variability in the pharmacokinetics of the agents. The aim of this study was to establish the pharmacokinetics of cyclophosphamide, thiotepa, carboplatin and all relevant metabolites in a widely used high-dose combination and to study possible relationships between the pharmacokinetics and toxicity.

PATIENTS AND METHODS

Blood samples were collected from patients treated with modifications of the CTCb regimen consisting of cyclophosphamide (1000-1500 mg/m2/day), carboplatin (265-400 mg/m2/day) and thiotepa (80-120 mg/m2/day) as short infusions for four consecutive days. Thiotepa and its main metabolite tepa, ultrafilterable carboplatin, cyclophosphamide and its activated metabolites 4-hydroxycyclophosphamide and phosphoramide mustard were determined. Pharmacokinetics were assessed with the use of population pharmacokinetic analyses. Relationship between the area under the concentration-time curves (AUCs) of these compounds and toxicity were tested.

RESULTS

A total of 46 patients (83 courses of chemotherapy) was included. Relationships were identified between elevation of transaminases and the thiotepa and tepa AUC, mucositis and the tepa AUC and ototoxicity and the carboplatin AUC. A strong trend between the 4-hydroxycyclophosphamide AUC and veno-occlusive disease was found.

CONCLUSIONS

The complex pharmacokinetics of the different agents and their metabolites have been established and several relationships between the pharmacokinetics and toxicity were identified. These findings may form the basis for further treatment optimisation and dose-individualisation in this high-dose chemotherapy combination.

The clinical pharmacology of alkylating agents in high-dose chemotherapy

Alkylating agents are widely used in high-dose chemotherapy regimens in combination with hematological support. Knowledge about the pharmacokinetics and pharmacodynamics of these agents administered in high doses is critical for the safe and efficient use of these regimens. The aim of this review is to summarize the clinical pharmacology of the alkylating agents (including the platinum compounds) in high-dose chemotherapy. Differences between conventional and high doses will be discussed.

Clinical pharmacokinetics and dose optimisation of carboplatin

Carboplatin shares some of the therapeutic advantages of cisplatin, but without a significant incidence of the dose-limiting neurotoxicity and nephrotoxicity which is experienced with cisplatin. However, its use is associated with dose-limiting bone marrow suppression. Carboplatin is present in the blood as 3 distinct species. These are total platinum and 2 unbound species, carboplatin itself and a decarboxylated platinum-containing degradation product. The 2 main methods used to assay the unbound species are flameless atomic absorption spectrophotometry and high performance liquid chromatography. The first of these methods assays both unbound platinum species, the second is specific for carboplatin. Both unbound species have similar pharmacokinetic profiles for the first 12 hours post-dose. Carboplatin appears to have a linear pharmacokinetic profile over the doses used clinically and does not interact significantly with drugs that are used commonly in combination chemotherapy. The pharmacokinetics of carboplatin are adequately described by an open 2-compartment model with elimination from the central compartment. Its clearance is proportional to the glomerular filtration rate and the volume of distribution of the central compartment appears to correlate with extracellular fluid volume. The elimination half-life varies with renal function and is typically between 2 and 6 hours in patients with a normal glomerular filtration rate and may be as long as 18 hours in patients with impaired renal function. Relationships between systemic exposure to carboplatin, described as the area under the concentration-time curve (AUC), and both toxicity and response have been described. For toxicity the strongest evidence exists for a relationship between AUC and thrombocytopenia. To a lesser extent the relationship between AUC and neutropenia has also been described. Patients already treated with platinum analogues have been shown to develop a greater degree of myelosuppression from any given AUC. In addition, some evidence suggests a relationship between the shape of the concentration-time curve and myelotoxicity, where constant infusions appear less likely to cause myelosuppression on a mg/m2 dose administration basis. The relationship between AUC and response rate is not as clear, this may be related to the lack of studies describing both the dose and AUC of carboplatin. There appears to be a more clearly defined AUC-response relationship for ovarian cancer than for other malignancies, with an AUC of between 5 and 7 mg/ml.min being associated with the maximal response rate [located at the plateau on an AUC-response curve]. However, new data suggest that higher AUCs may lead to greater response rates. Data from testicular cancer also strongly supports an AUC-response relationship with an increased number of treatment failures with carboplatin AUCs < 5 to 6 mg/ml.min. Given the AUC-effect relationships described above a number of studies have been performed to develop models to describe the relationship between both dose and AUC and dose and platelet nadir. In adults, perhaps the most common method is that of Calvert which describes the relationship between dose and AUC. Paediatric formulas have also been described. More recently a number of limited sampling strategies have been proposed as well as Bayesian dose individualisation techniques.