Comparison of the pharmacokinetics and protein binding of the anticancer drug, amsacrine and a new analogue, N-5-dimethyl-9-[(2-methoxy-4-methylsulfonylamino)phenyl-amino] -4-acridinecarboxamide in rabbits

Physicochemical characterization of asulacrine towards the development of an anticancer liposomal formulation via active drug loading: stability, solubility, lipophilicity and ionization

To facilitate the development of a liposomal formulation for cancer therapy, the physicochemical properties of asulacrine (ASL), an anticancer drug candidate, were characterized. Nano-liposomes were prepared by thin-film hydration in conjugation with active drug loading using ammonium sulphate and post-insertion with Poloxamer 188. A stability-indicating HPLC assay with diode array detection was developed for the determination of ASL concentrations. The U-shaped pH-solubility profile in aqueous solutions, with a lowest solubility at pH 7.4 (0.843 μg/mL), indicated that ASL is an ampholyte, and dilution or neutralization of acidic drug solutions used in clinical trials with physiological fluids may cause drug precipitation. The basic pKa value measured by absorbance spectroscopy was 6.72. The logD value at pH 3.8 was 1.15 which increased to 3.24 as pH increased to 7.4. ASL was found to be the most stable in acidic conditions and degraded most rapidly in alkaline conditions. An extra-liposomal pH of 5.6 during drug loading was found to be optimal to achieve the highest drug loading (DL) of 4.76% and entrapment efficiency (EE) of 99.9%. At this pH, >90% of ASL was ionized conferring high drug solubility (1mg/mL) and acted as a reservoir of unionized ASL to be transported into liposomal cores. As a suspension the optimized liposomes showed great physicochemical stability for five months at 4°C. In summary, the obtained physicochemical parameters provided insightful information useful to maximise DL into the liposomes, and explain a tendency of drug precipitation of pH-solubilized formulations following intravenous infusion.

Prediction of pharmacokinetics prior to in vivo studies. 1. Mechanism-based prediction of volume of distribution

In drug discovery and nonclinical development the volume of distribution at steady state (V(ss)) of each novel drug candidate is commonly determined under in vivo conditions. Therefore, it is of interest to predict V(ss) without conducting in vivo studies. The traditional description of V(ss) corresponds to the sum of the products of each tissue:plasma partition coefficient (P(t:p)) and the respective tissue volume in addition to the plasma volume. Because data on volumes of tissues and plasma are available in the literature for mammals, the other input parameters needed to estimate V(ss) are the P(t:p)'s, which can potentially be predicted with established tissue composition-based equations. In vitro data on drug lipophilicity and plasma protein binding are the input parameters used in these equations. Such a mechanism-based approach would be particularly useful to provide first-cut estimates of V(ss) prior to any in vivo studies and to explore potential unexpected deviations between sets of predicted and in vivo V(ss) data, when the in vivo data become available during the drug development process. The objective of the present study was to use tissue composition-based equations to predict rat and human V(ss) prior to in vivo studies for 123 structurally unrelated compounds (acids, bases, and neutrals). The predicted data were compared with in vivo data obtained from the literature or at Roche. Overall, the average ratio of predicted-to-experimental rat and human V(ss) values was 1.06 (SD = 0.817, r = 0.78, n = 147). In fact, 80% of all predicted values were within a factor of two of the corresponding experimental values. The drugs can therefore be separated into two groups. The first group contains 98 drugs for which the predicted V(ss) were within a factor of two of those experimentally determined (average ratio of 1.01, SD = 0.39, r = 0.93, n = 118), and the second group includes 25 other drugs for which the predicted and experimental V(ss) differ by a factor larger than two (average ratio of 1.32, SD = 1.74, r = 0.42, n = 29). Thus, additional relevant distribution processes were neglected in predicting V(ss) of drugs of the second group. This was true especially in the case of some cationic-amphiphilic bases. The present study is the first attempt to develop and validate a mechanistic distribution model for predicting rat and human V(ss) of drugs prior to in vivo studies.

The effect of cimetidine, phenobarbitone and buthionine sulphoximine on the disposition of N-5-dimethyl-9-[(2-methoxy-4-methyl-sulphonylamino)phenylamino]- 4-acridinecarboxamide (CI-921) in the rabbit

N-5-dimethyl-9-[(2-methoxy-4-methylsulphonyl-amino)phenylamino]-4- acridinecarboxamide (CI-921) is an amsacrine analogue currently undergoing phase II clinical trials as an antitumor drug. Significant alterations in the plasma clearance (CL) of amsacrine have been demonstrated in rabbits after pretreatment with cimetidine (CT), phenobarbitone (PB) and buthionine sulphoximine (BSO). In the present study, the influence of these agents on the disposition of CI-921 was investigated in rabbits. After a short infusion of CI-921 (12.7 mumol/kg), blood (8 x 3 ml) was collected up to 12 h and the total plasma concentration of CI-921 determined by HPLC. Model-independent pharmacokinetic parameters were compared by Student's paired t-test. CT pretreatment significantly (P = 0.011) increased the AUC (mean, 21%; range, 3%-43%) and significantly (P = 0.019) decreased the CL (mean, 17%; range, 4%-30%). The induction effect of PB pretreatment was not confirmed with CI-921. No significant reduction in AUC or increase in CL was apparent. BSO pretreatment caused a small but significant (P = 0.049) increase in AUC (mean, 20.5%; range, 4%-59%) but had no effect on CL. Although more modest changes in kinetic parameters were observed with CI-921 than with amsacrine, these results suggested the involvement of the hepatic mixed-function oxidase system but not PB-inducible cytochrome P-450 isozymes in the elimination of CI-921 in the rabbit. As with amsacrine, a reduction in hepatic glutathione (GSH) concentrations in the body also appeared to have a modest effect on the disposition of CI-921.

The clinical pharmacokinetics of N-5-dimethyl-9-[(2-methoxy-4-methyl-sulfonylamino)phenylamino]-4 -acridinecarboxamide (CI-921) in a phase 1 trial

The pharmacokinetics of CI-921 were studied after 65 infusions over a 20-fold dose range (13-270 mg/m2 per day) in 16 patients during a phase 1 trial. CI-921 was given by a 15 min infusion on three consecutive days. Plasma samples were collected after the first and third infusions, and urine, at 6 h intervals throughout the 3 days. CI-921 concentrations were measured by an HPLC method. Maximum plasma concentrations ranged from 3-86 mumol/l. The plasma concentration-time disposition curves were mainly biphasic over the 24-h postinfusion period. There was no significant difference by the paired t-test between the Cmax, AUC, CL, Vss, MRT, t1/2 alpha, or t1/2 beta calculated for the first and third infusions. The means (range) of model-independent pharmacokinetic parameters were: CL, 158 (94-290) ml/h per kg; Vss, 319 (219-614) ml/kg; MRT, 2.1 (1.1-3.5) h; t1/2 alpha, 0.5 (0.2-1.1) h; and t1/2 beta, 2.6 (1.1-5.0) h. There was a strong linear correlation between the dose and the AUC and Cmax, suggesting linear kinetics over this dose range. A very small amount (less than 1%) of the total dose was excreted as unchanged CI-921 in the urine, mostly in the 12-h postinfusion period.