Carboplatin (Cb) clearance (Cl) in elderly and young patients with advanced non-small cell lung cancer (ANSCLC): An exploratory analysis of the Calvert’s formula

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Background: It seems that elderly lung cancer patients (p) respond better to combination chemotherapy than to single-agent therapy. Application of pharmacokinetic (pk) modelling to the routine therapeutic drug monitoring in elderly p help to individualize the dosage of anticancer drugs. The aim of this study was to explore the Calvert′s formula usefulness for calculating the Cb dose in elderly p by means of the predicted AUC/actual AUC ratio. Creatinine (Cr) Cl was calculated by Cockdroft-Gault equation. Methods: Between December 2005 and September 2006, 20 chemonaive ANSCLC p were included in two groups: 10 adults p ≤ 65 years and 10 aged p ≥ 75 years. Treatment consisted in Cb day 1 and gemcitabine 1250 mg/m2 days 1 and 8 every 21 days. Cb dose was calculated for a foreseen AUC = 5 in young adults and AUC = 4 in aged p. Three blood samples were collected at 1–2 hours(h), 3–5 h, 12–24 h postinfusion. Total and ultrafiltrated Cb was determined using a flameless atomic absortion spectrometer. For pk analysis non lineal effects models implemented in NONMEM program were used. Individual pk parameters were calculated and predictors in the structural model were analized. Results: Table 1 shows mean and variation coefficient of some biometric characteristics. Pk parameters analysis: ClAUC5 p = 137 mL/min; Clfactor (parameter for measuring Cl changes according to age) = 0.44; Cl AUC4 p = 59.7 mL/min. Cb Cl in elderly p was 43.55% with respect to younger p. In younger p estimated Cb Cl was 5.69% higher than Cb Cl calculated using the Calvert equation. In elderly p, the opposite effect was achieved, estimated Cb Cl was 26.85% lower than Cb Cl calculated Conclusions: The Cb Cl calculation by means of Calvert formula showed underestimation of the dose in younger p and upperestimation of the dose in elderly p. It is necessary to investigate the covariates which contribute to take away the strength of the Calvert equation, which could to affect the efficacy and toxicity of Cb.

[Table: see text]

No significant financial relationships to disclose.

Bioavailability and Pharmacokinetic Model for Ritonavir in the Rat

The aim of this study is to investigate in vivo the oral bioavailability of ritonavir and to evaluate the pharmacokinetic model that best describes the plasma concentration behavior after oral and intravenous administration. Male Wistar rats were intravenously administered at 3 mg dose of pure ritonavir and oral administered at 4.6 +/- 2.5 mg of diluted Norvir. Blood samples were taken by means of the jugular vein for a 24 h period of time. An analytical high-performance liquid chromatography (HPLC) technique was developed in order to quantify ritonavir plasma concentrations. A nonlinear modeling approach was used to estimate the pharmacokinetic parameters of interest. Results showed that a two-compartmental model with zero-order kinetic in the incorporation process of ritonavir into the body better fitted intravenous and oral data. The estimated oral bioavailability by means of noncompartmental and compartmental approaches resulted in 74% and 76.4%, respectively. These values confirm the ones obtained by other authors in the rat. In conclusion, a zero-order kinetic in the incorporation process at the administered doses suggests the saturation of the possible specialized transport mechanisms involved in the incorporation of ritonavir into the body. These results could justify the use of low doses of ritonavir when improving the bioavailability of other protease inhibitors (PIs) is required.

Labetalol absorption kinetics: Rat small intestine and colon studies

Labetalol is a widely used drug for the management of hypertension, which is preferably administered by the oral route despite its low bioavailability. The objective of this study is to ascertain the mechanisms underlying its absorption as an approach to help in predicting the influence of dosage changes, possible drug-drug and drug-fruit juice interactions. Perfusion experiments have been performed in rats in two sites of absorption: the intestine and the colon. The nonlinearity of the process has been established by means of the assay of a wide range of concentrations (2-2000 microM). Fitting of the concentration versus time data allows the estimation of passive diffusion constant in the intestine (1.42 +/- 0.05/h) and the colon (1.13 +/- 0.06/h), V(m) and K(m) of the input process (9.85 +/- 4.98 microM/h, and 10.44 +/- 26.16 microM, respectively) and K(m) of an efflux system (0.53 +/- 1.16 microM) and V(m) in both intestinal segments (2.60 +/- 11.37 microM . /h in the intestine and 0.66 +/- 1.38 microM . /h in the colon). The efflux carrier implicated is identified by means of several inhibition experiments, whose inhibition ability is mathematically estimated. Results suggest the p-glycoprotein as responsible for the efflux of labetalol.

Modelling intestinal absorption of salbutamol sulphate in rats

The objective was to develop a semiphysiological population pharmacokinetic model that describes the complex salbutamol sulphate absorption in rat small intestine. In situ techniques were used to characterize the salbutamol sulphate absorption at different concentrations (range: 0.15-18 mM). Salbutamol sulphate at concentration of 0.29 mM was administered in presence of verapamil (10 and 20 mM), grapefruit juice and sodium azide (NaN3) (0.3, 3 and 6 mM). Different pharmacokinetic models were fitted to the dataset using NONMEM. Parametric and non-parametric bootstrap analyses were employed as internal model evaluation techniques. The validated model suggested instantaneous equilibrium between salbutamol sulphate concentrations in lumen and enterocyte, and the salbutamol sulphate absorption was best described by a simultaneous passive diffusion (ka = 0.636 h(-1)) and active absorption (VMax = 0.726 mM/h, Km = 0.540 mM) processes from intestinal lumen to enterocyte, together with an active capacity-limited P-gp efflux (V'max = 0.678 mM/h, K'm = 0.357 mM) from enterocyte to intestinal lumen. The extent of salbutamol sulphate absorption in rat small intestine can be improved by NaN3, grapefruit juice and verapamil.

Kinetic modelling of the intestinal transport of sarafloxacin. Studies in situ in rat and in vitro in Caco-2 cells

The absorption kinetics of sarafloxacin, as a model of fluoroquinolone structure, were studied in the rat small intestine and in Caco-2 cells. The objective of the study was to investigate the mechanistic basis of the drug's intestinal transport in comparison with other members of the fluoroquinolone family and to apply a mathematical modelling approach to the transport process. In the rat small intestine, sarafloxacin showed dual mechanisms of intestinal absorption with a passive diffusional component and an absorptive carrier-mediated component. The characteristics of the animal study design made it suitable for population analysis, thus allowing the accurate estimation of transport parameters and their inter and intra-individual variances. The transport system in the rat model was ATP-dependent, as sodium azide was able to decrease the absorption rate constant in a concentration-dependent fashion. The inhibition mechanism of sodium azide was modelled based on its ATP depletion capacity. The rationale of this approach was to consider the inhibitor-carrier interaction as a concentration- dependent response. This interaction was accurately described by a non-competitive mechanism. In Caco-2 cells, sarafloxacin showed a concentration dependent permeability in both directions apical to basal, and basal to apical. The permeability values and ratios of permeability values at different concentrations suggested the presence of two carriers (absorption and efflux carriers). The passive diffusion component in both systems was compared to that predicted by the absorption-partition correlation, previously established for two series of fluoroquinolones. The discrepancy between the experimental and predicted value suggested the presence of an efflux mechanism similar to that already described for other fluoroquinolones. The differences and similarities of the in situ and the in vitro results are discussed as well as the usefulness of the modelling approach.

Use of nonlinear mixed effect modeling for the intestinal absorption data: Application to ritonavir in the rat

The aim of this study is to investigate in situ the mechanisms involved in the gastrointestinal absorption of ritonavir in the rat, as an animal model for preclinical studies of anti-HIV agents in vivo. Four ritonavir solutions (40, 27, 13 and 7 microM) in the presence of 1% dimethylsulfoxide (DMSO) were perfused in the small intestine of anaesthetised rats. Effects of DMSO on the intestinal permeability were investigated using solutions containing antipyrine 1.33 mM and ritonavir 7 microM with and without 1% of DMSO. Antipyrine and ritonavir transport was not modified in the presence of 1% of DMSO. The population pharmacokinetic parameters of the ritonavir intestinal transport were obtained by means of nonlinear mixed effect modelling approach according to a nonlinear absorption and nonlinear secretion. The absorption and secretion kinetic parameters for ritonavir were: Vm=47.6 microM/h; Km=8.77 microM; Vms=3.66 microM/h and Kms=0 microM. The interindividual variability found to ritonavir Vm 13.1%, and the residual variability was 8.98%. The Kms value support the saturation of the carrier at the range of concentrations of ritonavir assayed. The interindividual variability value of the Vm could explain, at least in part, the variability in absorption rate constants observed.

Pharmacokinetic models for the saturable absorption of cefuroxime axetil and saturable elimination of cefuroxime

Since oligopeptidic drugs such as beta-lactam antibiotics share the same carriers in humans and animals, the absorption and elimination kinetics of cefuroxime (C) were investigated in rats. Plasma C concentrations were measured by liquid chromatography. Pharmacokinetics and bioavailability of C in the rat were examined after intravenous (i.v.) administration at three doses (1.78, 8.9 and 17.8mg) of cefuroxime sodium and oral administration at two doses (2.02 and 8.9mg) of cefuroxime axetil (CA). Preliminary fits using data from intravenous administration of C showed that the drug disposition kinetics were clearly nonlinear, with an increase in plasma clearance as the intravenous dose increased. After oral administration of CA, normalized C(max) was higher for smaller dose than for the largest dose. The population pharmacokinetic parameters were obtained by means of nonlinear mixed effect modelling approach according to a nonlinear elimination and nonlinear absorption two-compartment model. The nonlinear elimination could be attributed to a saturable renal tubular reabsorption of the antibiotic and nonlinear intestinal absorption of CA mediated by carrier system. The oral bioavailability of C, calculated by numeric integration of an amount of CA drug absorbed was 22 and 17% for 2.02 and 8.9mg of prodrug administered orally.

The influence of active secretion processes on intestinal absorption of salbutamol in the rat

Salbutamol was perfused in the small intestine of rat using a standard rat gut "in situ" preparation: (1) in inhibitor-free solution at seven different concentrations (0.15, 0.29, 1.20, 5.0, 9.0, 13.0 and 18.0mM); (2) at a 0.29mM concentration - thought to be close to the allometric dose in man - in the presence of a non-specific enzyme inhibitor, sodium azide (0.3, 3.0 and 6.0mM); and (3) at 0.29mM in the presence of a selective secretion inhibitor, verapamil (10.0 and 20.0mM). In free solution, the mixed-order rate constants, k'(a), of salbutamol increase as the solute concentration increases until an apparent asymptotic value is reached. This could be due to the saturation of enzymatic systems responsible for the secretion of the drug from the enterocyte to the luminal fluid, a process that could explain the poor absorption of salbutamol. In the presence of sodium azide, the k(a) values increased about 1.5-fold, whereas in the presence of verapamil they increased two- to three-fold. These results indicate that salbutamol can act as a substrate of an intestinal secretory transport, which probably includes--at least in part--the enzyme P-glycoprotein, since verapamil has been shown to inhibit this enzyme by dose-dependent competition. This leads to a secretion-limited peroral absorption of salbutamol, which contributes to the poor oral bioavailability of the drug. The possible options for improving salbutamol absorption are discussed.

Intestinal absorption kinetics of amiodarone in rat small intestine

Amiodarone is a widely used antiarrhythmic agent with highly variable therapeutic effects. These seem to be related, at least in part, to the pharmacokinetics of the drug and particularly to some features of its gastrointestinal absorption process. The drug exhibits physico-chemical properties highly suitable for diffusion across lipophilic absorbing membranes, but its low aqueous solubility can act as the rate limiting step for absorption, making the process erratic and variable. In order to gain an insight into the intestinal absorption mechanism of the drug and detect possible non-linearities, a series of experiments using a classical rat gut in situ preparation were carried out with three amiodarone hydrochloride solutions (10, 75, and 200 micrograms mL-1). A synthetic non-ionic surfactant, polysorbate 80, at supramicellar concentration (2 mM) was used as the drug solubilizer. Amiodarone was assayed in biological samples by HPLC using a rapid, sensitive technique that was validated. The amiodarone first-order absorption rate constants obtained in these conditions were similar. No significant differences between ka values were found. Amiodarone absorption was clearly identified as a passive diffusion process.

Interaction of taurine on baclofen intestinal absorption: a nonlinear mathematical treatment using differential equations to describe kinetic inhibition models

Previous studies showed that the in situ absorption of baclofen in rat jejunum was inhibited by beta-alanine, a nonessential amino acid, and therefore mediated, at least in part, by some beta-amino acid carrier. In this paper a similar study was undertaken using taurine, a sulfonic beta-amino acid, in order to evaluate its effect and to establish a general inhibition model. To achieve this goal, remaining concentrations of inhibitor were also measured and incorporated into the model. Previously, kinetic absorption in situ parameters for taurine in free solution were obtained: Vm = 27.73 +/- 9.99 mM h-1, K(m) = 8.06 +/- 2.82 mM, Ka (passive difussion component) = 0.40 +/- 0.28 h-1. Isotonic solutions containing 0.5 mM baclofen with starting taurine concentrations ranging from 0 to 100 mM were perfused in rat jejunum, and the remaining concentrations of both compounds were measured. The apparent rate pseudoconstant of the drug clearly decreased as the remaining taurine concentration increased. The interaction can be described as a complete competitive inhibition plus a second component, K, noninhibited, K = 0.58 (+/- 0.03) h-1, Ki = 20.62 (+/- 4.04) mM, Vmi = 28.12 (+/- 6.12) mM h-1, Kmi = 11.71 (+/- 2.53) mM, Kai = 0.47 (+/- 0.10) h-1. A residual absorption of baclofen in the presence of high taurine concentrations was observed, which should be attributed to another transport system not associated with the taurine carrier. In order to elucidate whether or not taurine and beta-alanine carriers are two separate entities that baclofen can use for absorption, further experiments using beta-alanine and taurine together as inhibitors (baclofen, 0.5 mM; beta-alanine, 50 mM, and taurine, 50 mM) were developed. Results indicated that baclofen and both amino acids share the same carrier in the intestinal absorption process. We have completed studies using leucine, taurine, and GABA together as inhibitors of drug absorption. An isotonic perfusion solution of 0.5 mM baclofen in the presence of 50 mM leucine, 25 mM taurine, and 25 mM GABA was perfused. Under these conditions the absorption rate pseudoconstant of baclofen decreases until 0.080 h-1 (+/- 0.069). Practical implications of these phenomena are briefly discussed.

Influence of leucine on intestinal baclofen absorption as a model compound of neutral alpha-aminoacids

The inhibitory effect of the essential alpha-aminoacid L-leucine on the intestinal absorption of the antispastic drug baclofen was examined by means of an in situ rat gut perfusion technique. When 0.5 mM baclofen solutions were perfused in the presence of increasing concentrations of the aminoacid (5-100 mM), the apparent absorption rate constant of the drug decreased as the initial leucine concentration increased. Higher leucine concentrations however did not completely abolish the absorption of the drug (at 100 mM of leucine, only 76% inhibition was observed). The interaction can be mathematically described as a complete competitive inhibition with a second component, K = 0.35 (+/- 0.08)h-1, Ki = 0.25 (+/- 0.09)mM, AIC = -97.02. In the light of some of the absorption features of the drug, however, the residual absorption of baclofen in the presence of high leucine concentrations should be attributed to another transport system not used by leucine. Apparent parameters characterizing absorption of leucine in the presence of baclofen (0.5 mM) were Vm = 61.02 (+/- 5.46)mM h-1; Km = 8.04 (+/- 0.89)mM, and AIC = -62.25. The results indicate that baclofen and leucine share some carriers in the intestinal absorption processes. Since leucine is an essential dietary aminoacid, and therefore a normal food component, this finding could be relevant in preventing interactions that would lead to a reduced oral bioavailability during baclofen therapy.

Experimental studies on the influence of surfactants on intestinal absorption of drugs. Cefadroxil as model drug and sodium lauryl sulfate as model surfactant: studies in rat duodenum

The effect of sodium lauryl sulfate (CAS 151-21-3) on the duodenal absorption of cefadroxil (CAS 50370-12-2) has been investigated with the aid of a classical rat gut in situ preparation. Both compounds were entirely compatible in working solutions. Cefadroxil was found to be very stable and only slightly solubilized in the micellar phase. The apparent first-order absorption rate constants for the free antibiotic fraction were determined in free solution, and in the presence of variable surfactant concentration in luminal fluid. A functional interpretation of these data, based both on the law of mass action and the complete noncompetitive transport inhibition equations, showed that the surfactant acts as a nonspecific inhibitor of the carrier-mediated absorption of the antibiotic, but also as an enhancer of its passive absorption component. The net result was an outstanding reduction in the absorption capacity of cefadroxil when it was perfused at 0.1 mg/ml, i.e. far from its carrier saturation (from 3.0 h-1 in free solution to 2.0(-1) at high surfactant concentration, with a minimum of about 1.4 h-1 in the presence of the surfactant at 0.5 mg/mg in duodenal fluid). When cefadroxil was perfused at 10.0 mg/ml, i.e. with its carrier-mediated transport beyond the saturation, the net result was a progressively enhanced absorption (ranging from about 0.9 h-1 in free solution to 2.0 h-1 at high surfactant concentration).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of surfactants on amiodarone intestinal absorption. I. Sodium laurylsulfate

Amiodarone is a widely used antiarrhythmic agent with high variability in therapeutic effects, which appears to be related, at least in part, to its pharmacokinetics, and in particular, gastrointestinal absorption. The drug exhibits physico-chemical properties highly suitable for diffusion across lipophilic absorbing membranes but its low aqueous solubility can act as the rate limiting step for absorption, making it erratic and variable. In studying the intestinal absorption mechanism of amiodarone, a series of experiments using a rat gut in situ preparation was performed in the presence of a synthetic anionic surfactant, as a drug solubilizer, i.e., sodium laurylsulfate, at variable supramicellar concentrations (from 2.6 to 104 mM). Absorption rate constants of amiodarone decreased as surfactant concentration increased, the absorption being unusually fast at lower surfactant concentrations. Equations were developed to evaluate the relationship between absorption rate constant and surfactant concentration in the intestinal luminal fluid.

Studies on the reliability of a novel absorption-lipophilicity approach to interpret the effects of the synthetic surfactants on drug and xenobiotic absorption

Some theoretical principles of the absorption/lipophilicity approach, which attempts to explain the effects of the synthetic surfactants on xenobiotic and drug intestinal absorption, are reviewed and experimentally checked by examining the correlations obtained between "in situ" absorption constants, ka, found in rat colon, and "in vitro" lipophilicity indexes, K', for two compound series (secondary aliphatic amines and phenylalkylamines) in the absence and in the presence of the nonionic surfactant Polysorbate 80, in the intestinal perfusion fluid. Evidence is given for the following actions of the synthetic surfactant: at its critical micelle concentration (CMC), it increases the polarity of the absorbing membrane and, at the same time, it disrupts the aqueous stagnant diffusion layer adjacent to the mucosal barrier. When a supramicellar concentration (SMC) is used, the above actions are almost totally masked by the micellar solubilization of the tested amines, which decreases their absorption constants relative to those found at CMC, as markedly as solute lipophilicity increases. As a consequence of these actions, the correlations between ka and K', which are clearly hyperbolic in free solution, become potential in the presence of the surfactant at its CMC, whereas at SMC a bilinear correlation is obtained. Absorption via lipophilic ionized species seems to take place for both compound series. Mathematical and physicochemical interpretations of this behaviour are outlined, and biopharmaceutical implications of these phenomena are discussed.

Evidence of a specialized transport mechanism for the intestinal absorption of baclofen

Absorption of the spasmolytic drug baclofen in three selected intestinal segments of living anaesthetized rats in situ, is shown to be a specialized transport mechanism obeying Michaelis-Menten kinetics. Equation parameters were calculated through different procedures, whose features are discussed. A computer method based on the integrated form of Michaelis-Menten equation which reproduces the entire time course of drug absorption from the data found in three intestinal perfusion series at different initial concentrations, yielded Vm and Km values of 12.0 mg h-1 and 8.0 mg, respectively, in the mean segment of the small intestine, a rather selective absorption site for baclofen. Lesser but comparable absorption rates were found in the proximal and distal segments of the small intestine, whereas in colon, drug absorption was negligible. Baclofen transport was significantly reduced in the presence of the enzymatic inhibitor sodium azide. If these results were extrapolated to humans, they would explain the excellent bioavailability profiles reported for baclofen at normal doses in spite of its physicochemical properties, which do not favour passive diffusion. Based on the same principle, the administration of usual doses at shorter time intervals could be recommended, instead of high, when higher plasma levels at steady-state are needed. On the other hand, more than 8-h sustained-release preparations of baclofen should, probably, be avoided.

Non-linear intestinal absorption kinetics of cefadroxil in the rat

Absorption of cefadroxil in a selective intestinal absorption area (the proximal third of the small intestine) of the anaesthetized rat, at seven initial perfusion concentrations, ranging from 0.01 to 10.0 mg mL-1, is shown to be a non-linear transport mechanism. With the aid of computer-fitting procedures based on differential and integrated forms of Michaelis-Menten equation, Vm and Km values of 36.7-37.3 mg h-1 and 12.0-13.0 mg, respectively, were found. The statistical parameters were better than those obtained both for first-order and for combined Michaelis-Menten and first-order kinetics. There is no evidence for substantial passive diffusion processes. The results reported here, together with allometric considerations and literature data analysis, may help to explain some particular non-linear features of plasma level curves associated with the administration of fairly high oral doses of cefadroxil to humans.

Studies on the reliability of a bihyperbolic functional absorption model. II. Phenylalkylamines

Evidence is given that demonstrates the reliability of the bihyperbolic equation, proposed by Plá-Delfina and Moreno, in fitting the correlation between absorption rate constants (ka) found in the small intestine and in the colon of the living anesthetized rat, and partition constants (1/RF-1), for a series of phenylalkylamines, a group of compounds which differ largely from others which have been tested. Emphasis is laid on the nonexistence of an optimum of lipophilicity for intestinal absorption/partition correlation: This feature makes inapplicable the probabilistic approaches to the reported data.