Effects of cyclophosphamide related genetic variants on clinical outcomes of adult hematopoietic cell transplant patients

Impact of dosing strategy on clinical outcomes of patients with lupus nephritis initially treated with lower-than-recommended-dose cyclophosphamide

AIM

Cyclophosphamide is the mainstay treatment for patients with lupus nephritis (LN); it can be prescribed at lower doses than the recommended regimen to avoid side effects. We aimed to investigate the impact of cyclophosphamide dosing strategies on treatment outcomes of patients with LN initially treated with a lower-than-recommended dose.

METHODS

We retrospectively reviewed patients with proliferative LN (class III, IV, or mixed) initially treated with lower-than-recommended-dose cyclophosphamide. Patients who received a titrated dose of cyclophosphamide ≥0.5 g/m2 were categorized into the titrate group, while those who received doses <0.5 g/m2 were categorized into the non-titrate group. The primary outcome was primary renal response (PRR) at 52 weeks.

RESULTS

Of the 78 patients included, 47 were assigned to the titrate group and 31 to the non-titrate group. The titrate group had a higher proportion of PRR achievement (23 of 47 patients [48.9 %] vs. 7 of 31 patients [22.6 %] in the non-titrate group). After adjusting for potential confounders, a baseline urinary protein-to-creatinine ratio ≥3 g/g (OR, 0.3; 95 % CI, 0.1-0.9; P = 0.030), and titrating the dose of cyclophosphamide to ≥0.5 g/m2 (OR, 4.7; 95 % CI, 1.5-15.2; P = 0.010) were independent factors for PRR. Additionally, the titrate group had a lower rate of infection (8 of 47 patients [17.0 %] vs. 12 of 31 patients [38.7 %], respectively; OR, 0.3; 95 % CI, 0.1-0.9; P = 0.036) and death associated with LN (4 of 47 patients [8.5 %] vs. 8 of 31 patients [25.8 %], respectively; OR, 0.3; 95 % CI, 0.1-0.9; P = 0.047) compared with the non-titrate group. LN flare and the need for rescue therapy did not differ between the groups.

CONCLUSION

For patients with LN initially treated with lower-than-recommended-dose cyclophosphamide, titration of the cyclophosphamide dose ≥0.5 g/m2 was beneficial on renal response, while reducing infection leading to hospitalization and LN-associated death.

Cyclophosphamide Pharmacogenomic Variation in Cancer Treatment and Its Effect on Bioactivation and Pharmacokinetics

Cyclophosphamide (Cy) is a prodrug that is mainly bioactivated by cytochrome P450 (CYP) 2B6 enzyme. Several other enzymes are also involved in its bioactivation and affect its kinetics. Previous studies have shown the effect of the enzymes' genetic polymorphisms on Cy kinetics and its clinical outcome. These results were controversial primarily because of the involvement of several interacting enzymes in the Cy metabolic pathway, which can also be affected by several clinical factors as well as other drug interactions. In this review article, we present the effect of CYP2B6 polymorphisms on Cy kinetics since it is the main bioactivating enzyme, as well as discussing all previously reported enzymes and clinical factors that can alter Cy efficacy. Additionally, we present explanations for key Cy side effects related to the nature and site of its bioactivation. Finally, we discuss the role of busulphan in conditioning regimens in the Cy metabolic pathway as a clinical example of drug-drug interactions involving several enzymes. By the end of this article, our aim is to have provided a comprehensive summary of Cy pharmacogenomics and the effect on its kinetics. The utility of these findings in the development of new strategies for Cy personalized patient dose adjustment will aid in the future optimization of patient specific Cy dosages and ultimately in improving clinical outcomes. In conclusion, CYP2B6 and several other enzyme polymorphisms can alter Cy kinetics and consequently the clinical outcomes. However, the precise quantification of Cy kinetics in any individual patient is complex as it is clearly under multifactorial genetic control. Additionally, other clinical factors such as the patient's age, diagnosis, concomitant medications, and clinical status should also be considered.

Comparison of Dose Adjustment Strategies for Obesity in High-dose Cyclophosphamide Among Adult Hematopoietic Cell Transplantation Recipients: Pharmacokinetic Analysis

Cyclophosphamide (CY) is an alkylating agent widely used in the field of oncology and hematopoietic cell transplantation (HCT). It is recommended to use an adjusted body weight with an adjustment factor of 0.25 (ABW25) for dosing of CY in obese patients undergoing HCT. However, evidence based on the pharmacokinetics (PK) of CY to support this recommendation is lacking. We aimed to identify a dosing strategy of CY that achieves equivalent exposures among obese and nonobese patients. The present study is a secondary analysis of a previously conducted observational PK study of phosphoramide mustard (PM), the final cytotoxic metabolite of CY. Data were collected from 85 adults with hematologic malignancy who received a single infusion of CY 50 mg/kg, fludarabine, ± anti-thymocyte globulin, and a single fraction of total body irradiation as HCT conditioning therapy. A previously developed population PK model in these patients was used for simulations. Using individualized PK parameters from that analysis, simulations were performed to assess cumulative exposures of PM (i.e., area-under-the-curve [AUC]) resulting from 8 different dosing strategies according to various measures of body size: (1) "mg/kg" by total body weight (TBW); (2) "mg/kg" by ideal body weight (IBW); (3) "mg/kg" by fat free mass; (4) "mg/m²" by body surface area (BSA); (5) "mg/kg" by TBW combined with ABW25 (TBW-ABW25); (6) "mg/kg" by IBW combined with ABW25 (IBW-ABW25); (7) "mg/kg" by TBW combined with ABW by adjustment factor of 0.50 (TBW-ABW50); and (8) "mg" by fixed-dose. We defined equivalent exposure as the effect of obesity on PM AUC within ±20% from the PM AUC in the nonobese group, where obesity is defined based on TBW/IBW ratio (i.e., nonobese, <1.2; mildly obese, 1.2-1.5; and moderately/severely obese, >1.5). Primary and secondary outcomes were PM AUC_0-8hours and PM AUC_0-infinity, respectively. In the 85 patients, with the median age of 63 years (range 21-75), 46% were classified as mildly and 25% were moderately/severely obese based on the TBW/IBW ratio. Negative correlations (i.e., higher the extent of obesity, lower the PM AUC) were shown when dosing simulations were based on IBW, TBW-ABW25, and fixed dosing (P < .05). Positive correlations were shown when dosing was simulated by TBW (P < .05). None of the 8 dosing strategies attained equivalent PM AUC_0-8hours between patients with versus without obesity, whereas dosing by BSA and TBW-ABW50 attained equivalent PM AUC_0-infinity (P < .05). Our study predicted that the recommended ABW25 dose adjustment may result in lower exposure of CY therapy in obese patients than in nonobese. A CY dosing strategy that would result in similar PM concentrations between obese and nonobese was not identified for early exposure (i.e., PM AUC_0-8hours). The data suggest though that CY dosing based on "mg/m²" by BSA or "mg/kg" by TBW-ABW50 would result in similar total exposure (i.e., PM AUC_0-infinity) and may minimize exposure differences in obese and nonobese patients.