Population differences in S-warfarin pharmacokinetics among African Americans, Asians and whites: their influence on pharmacogenetic dosing algorithms.

Using population pharmacokinetic analysis (PPK), we attempted to identify predictors of S-warfarin clearance (CL(S)) and to clarify population differences in S-warfarin pharmacokinetics among a cohort of 378 African American, Asian and white patients. Significant predictors of CL(S) included clinical (age, body weight and sex) and genotypic (CYP2C9*2,*3 and *8) factors, as well as African American ethnicity, the median CL(S) being 30% lower in the latter than in Asians and whites (170 versus 243 and 250 ml h-1, P<0.01). The plasma S-warfarin (Cp(S)) time courses following the genotype-based dosing algorithms simulated using the PPK estimates showed African Americans with CYP2C9*1/*1 and any of the VKORC1 genotypes would have an average Cp(S) at steady state 1.5-1.8 times higher than in Asians and whites. These results indicate warfarin dosing algorithms should be evaluated in each respective ethnic population. Further study of a large African American cohort will be necessary to confirm the present findings.

Donepezil plasma concentrations, CYP2D6 and CYP3A4 phenotypes, and cognitive outcome in Alzheimer's disease.

PURPOSE: The purpose of the study is to evaluate whether donepezil (D) plasma concentrations and activity of CYP2D6 and CYP3A4 are associated with the therapeutic response of patients with mild to moderate Alzheimer's disease (AD). METHODS: This study comprised 54 patients affected by probable AD in therapy with D 10 mg/daily for at least 3 months. Plasma concentrations of D and its three main metabolites (6DD, 5DD, DNox) were assayed with a novel high performance liquid chromatography (HPLC) technique. Cognitive progression was assessed at baseline and at 9 months of follow-up with the mini mental state examination (MMSE). The activities of the two cytochromes involved in D metabolism-CYP2D6 and CYP3A4-were evaluated according to their metabolic ratios in plasma or urine, after test doses of probe drugs (dextromethorphan and omeprazole). RESULTS: A significant correlation was found between plasma levels of D and variations in MMSE scores after 9 months of therapy (r (2) = 0.14; p = 0.006). Neither the concentrations of D metabolites nor the metabolic ratios of CYP2D6 and CYP3A4 showed any correlations with cognitive variations. Low CYP2D6 activity and advanced age were associated with high D concentrations. Patients who were treated with CYP2D6 and P-glycoprotein (P-gp) inhibitors also had higher D plasma levels (mean difference = 19.6 ng/mL; p = 0.01) than those who were not. CONCLUSIONS: D plasma concentrations, but not cytochrome phenotyping, are associated with cognitive outcomes in AD patients.

A pharmacometric model describing the relationship between warfarin dose and INR response with respect to variations in CYP2C9, VKORC1, and age.

The objective of the study was to update a previous NONMEM model to describe the relationship between warfarin dose and international normalized ratio (INR) response, to decrease the dependence of the model on pharmacokinetic (PK) data, and to improve the characterization of rare genotype combinations. The effects of age and CYP2C9 genotype on S-warfarin clearance were estimated from high-quality PK data. Thereafter, a temporal dose-response (K-PD) model was developed from information on dose, INR, age, and CYP2C9 and VKORC1 genotype, with drug clearance as a covariate. Two transit compartment chains accounted for the delay between exposure and response. CYP2C9 genotype was identified as the single most important predictor of required dose, causing a difference of up to 4.2-fold in the maintenance dose. VKORC1 accounted for a difference of up to 2.1-fold in dose, and age reduced the dose requirement by ~6% per decade. This reformulated K-PD model decreases dependence on PK data and enables robust assessment of INR response and dose predictions, even in individuals with rare genotype combinations.

Predictors of survival and toxicity in patients on adjuvant therapy with 5-fluorouracil for colorectal cancer.

The present study aimed at investigating whether the simultaneous evaluation of pharmacokinetic, pharmacogenetic and demographic factors could improve prediction on toxicity and survival in colorectal cancer patients treated with adjuvant 5-fluorouracil (5FU)/leucovorin therapy. One hundred and thirty consecutive, B2 and C Duke's stage colorectal cancer patients were prospectively enrolled. 5FU pharmacokinetics was evaluated at the first cycle. Thymidylate synthase (TYMS) 5'UTR and 3'UTR polymorphisms and methylenetetrahydrofolate reductase (MTHFR) C677T and A1298C polymorphisms were assessed in peripheral leukocytes. Univariate and multivariate analyses were applied to evaluate which variables could predict chemotherapy-induced toxicity, disease-free survival (DFS) and overall survival (OS). Multivariate analysis showed that: (a) low 5FU clearance was an independent predictive factor for severe toxicity (OR=7.32; P<0.0001); (b) high-5FU clearance predicted poorer DFS (HR=1.96; P=0.041) and OS (HR=3.37; P=0.011); (c) advanced age was associated with shorter DFS (HR=3.34; P=0.0008) and OS (HR=2.66; P=0.024); (d) the C/C genotype of the MTHFR C677T polymorphism was protective against grade 3-4 toxicity (P=0.040); (e) none of the TYMS polymorphisms could explain 5FU toxicity or clinical outcome.

Von Willebrand factor propeptide makes it easy to identify the shorter Von Willebrand factor survival in patients with type 1 and type Vicenza von Willebrand disease.

Reduced von Willebrand factor (VWF) half-life has been suggested as a new pathogenic mechanism in von Willebrand disease (VWD). The usefulness of VWF propeptide (VWFpp) in exploring VWF half-life was assessed in 22 type 1 and 14 type Vicenza VWD patients, and in 30 normal subjects, by comparing the findings on post-Desmopressin (DDAVP) VWF t(1/2) elimination (t(1/2el)). The VWFpp/VWF antigen ratio (VWFpp ratio) was dramatically increased in type Vicenza VWD (13.02 +/- 0.49) when compared to normal subjects (1.45 +/- 0.06), whereas it appeared to be normal in all type 1 VWD patients (1.56 +/- 0.7), except for the four carrying the C1130F mutation (4.69 +/- 0.67). A very short VWF t(1/2el) was found in type Vicenza VWD (1.3 +/- 0.2 h), while all type 1 VWD patients had a t(1/2el) similar to that of the controls (11.6 +/- 1.4 and 15.4 +/- 2.5 h respectively), except for the four patients carrying the C1130F mutation, who had a significantly shorter VWF survival (4.1 +/- 0.2 h). A significant inverse correlation emerged between VWFpp ratio and VWF t(1/2el) in both VWD patients and normal subjects. The VWFpp ratio thus seemed very useful for distinguishing between type 1 VWD cases with a normal and a reduced VWF survival, as well as for identifying type Vicenza VWD.

A PK-PD model for predicting the impact of age, CYP2C9, and VKORC1 genotype on individualization of warfarin therapy.

The aim of this study was to characterize the relationship between warfarin concentrations and international normalized ratio (INR) response and to identify predictors important for dose individualization. S- and R-warfarin concentrations, INR, and CYP2C9 and VKORC1 genotypes from 150 patients were used to develop a population pharmacokinetic/pharmacodynamic model in NONMEM. The anticoagulant response was best described by an inhibitory E(MAX) model, with S-warfarin concentration as the only exposure predictor for response. Delay between exposure and response was accounted for by a transit compartment model with two parallel transit compartment chains. CYP2C9 genotype and age were identified as predictors for S-warfarin clearance, and VKORC1 genotype as a predictor for warfarin sensitivity. Predicted INR curves indicate important steady-state differences between patients with different sets of covariates; differences that cannot be foreseen from early INR assessments alone. It is important to account for CYP2C9 and VKORC1 genotypes and age to improve a priori and a posteriori individualization of warfarin therapy.

Pharmacokinetic and demographic markers of 5-fluorouracil toxicity in 181 patients on adjuvant therapy for colorectal cancer.

BACKGROUND: The relationship between 5-fluorouracil (5-FU) pharmacokinetics and toxicity following i.v. bolus administration has not been extensively studied. PATIENTS AND METHODS: One hundred and eighty-one patients on adjuvant therapy with 5-FU plus leucovorin for colorectal cancer were the study population. 5-FU pharmacokinetics was determined on day 2 of the first, third, and fifth cycles; type and the grade of adverse reactions were recorded on the next cycle. RESULTS: The 5-FU area under the curve (AUC) measured at the first cycle ranged between 146 and 1236 mg x min/l and was significantly correlated with drug dose, patients' body weight (BW) and gender, females having higher AUCs. These covariates explained only 23% of AUC variability. AUC and age were the only covariates which discriminated between toxic (grade > or =2) and nontoxic cycles (grade <2), with an optimal AUC cut-off value of 596 mg x min/l. Such a correlation was lost during the next cycles following dose reduction because of toxicity in 80 patients. CONCLUSIONS: A method for calculating the initial 5-FU dose is proposed which takes into account patient BW, gender and a target AUC of 596 mg x min/l. Nevertheless, it appears that a substantial part of 5-FU toxicity is not linked to pharmacokinetic factors and dose adjustments must still be on the basis of careful clinical surveillance.

A novel G/C single-nucleotide polymorphism in the double 28-bp repeat thymidylate synthase allele.

Thymidylate synthase (TYMS) is the main molecular target for fluoropyrimidine anticancer drugs, and its expression has been correlated with the number of repeats of a 28-bp sequence in the 5'-untranslated region of the TYMS gene and with the presence of a G --> C single-nucleotide polymorphism in the second repeat of 3R alleles. Based on this double polymorphism, three main TYMS alleles have so far been identified: TYMS 2R, TYMS 3RC and TYMS 3RG. During genetic analysis of TYMS polymorphisms in 100 colorectal cancer patients, three patients revealed an unexpected 113-bp band after electrophoresis of the restriction fragment length polymorphism analysis. Subsequent sequencing revealed two 28-bp repeats in the 5'-untranslated region and the presence in both repeats of cytosine instead of guanine at the 12th nucleotide. This allele variant (TYMS 2RC) has not been previously described in man. All three patients were heterozygotes for TYMS 2RC and experienced grade 2-3 chemotherapy-related toxicity.

Severe phenytoin intoxication in a subject homozygous for CYP2C9*3.

A 31-year-old woman who had a severe head injury was treated with oral phenytoin (100 mg 3 times a day) to prevent posttraumatic seizures. On day 10 of phenytoin treatment, 3 hours after the morning dose, the patient manifested neurologic signs compatible with phenytoin intoxication. Thus drug serum concentrations were monitored daily for 12 days. The elimination half-life was 103 hours, namely, about 5 times longer than the mean value generally quoted (22 hours). In the absence of any acquired predisposing factor for phenytoin toxicity, genetic mutations in the cytochrome P450 (CYP) enzymes responsible for phenytoin metabolism (CYP2C9 and CYP2C19) were suspected. Genotyping revealed that the patient was homozygous for the CYP2C9*3 allele (CYP2C9*3/*3) and heterozygous for the CYP2C19*2 allele (CYP2C19*1/*2). In view of the markedly reduced metabolic activity of CYP2C*3 in comparison with the wild-type enzyme (about one fifth) and of the minor role of CYP2C19 in phenytoin metabolism, it is likely that CYP2C9*3 mutation was largely responsible for drug overdose.

Morphological algebraic models of the TU-wave patterns/in idiopathic long QT syndrome.

A computer-assisted analysis of the TU-complex morphology was employed to characterize repolarization abnormalities in LQTS and to assess arrhythmic risk. Electrocardiograms (ECGs) were collected from 14 idiopathic LQTS patients (seven without symptoms and seven with a history of syncope or cardiac arrest) and from 14 sex- and age-matched normal subjects. Digitized TU-wave patterns from V2-V6 precordial leads were analyzed. The morphologies of the T and U waves were modeled by an algebraic sum of differences between two pairs of action potential-like curves of different shape and duration so that the whole TU complex was approximated by (S1-S2)+(L1-L2). By finding the best fit model of the digitized TU-wave signal, the amplitude and duration of each decomposition curve were determined for each lead. The following 'secondary' parameters were then derived: (a) the ratio between the sum of the amplitudes of the two long (L1 and L2) and the two short (S1 and S2) decomposition curves (A-ratio), (b) the highest A-ratio found in V2 to V6 (A-ratio(max)), and (c) the model-derived durations of the T-wave, U-wave and TU-complex. Conventional measures of RR and QTc intervals and of QT dispersion did not differ between symptomatic and asymptomatic LQTS patients. Modeled QT interval was significantly longer in the symptomatic than in the asymptomatic LQTS patients and in asymptomatic LQTS patients than in the controls. In addition, symptomatic LQTS patients had a longer S2 and T-wave duration in most leads than normal subjects. Conversely, modeled QU interval and U-wave duration did not significantly differ between the three groups. Compared to normal subjects, the amplitudes of S1, S2, L1 and L2 in the LQTS patients were not significantly different in most leads. A-ratio and A-ratio(max) were greater in symptomatic than asymptomatic LQTS patients and in the latter than in controls. A cut-off value of 0.90 of A-ratio(max) separated all symptomatic (1.34+/-0.38) from all asymptomatic patients (0.60+/-0.21). Although the correlation between model parameters and cellular substrate is at present unclear, it is possible that the morphological alterations described by the model are related to the arrhythmogenic mechanism(s) of the idiopathic LQTS.

Pharmacokinetics of intraperitoneal hyperthermic perfusion with mitoxantrone in ovarian cancer.

PURPOSE: Theoretical data and experimental assumptions indicate that intraperitoneal hyperthermic chemotherapy may play a role in the treatment of peritoneal carcinomatosis. The feasibility, tolerability and pharmacokinetics of intraperitoneal hyperthermic perfusion with mitoxantrone were studied in patients with pretreated ovarian cancer. METHODS: After cytoreductive surgery, 11 patients underwent intraperitoneal hyperthermic perfusion with mitoxantrone. A heated (42-43 degrees C) solution of the drug (28 mg/m2) was recycled through a perfusion apparatus into the abdominal cavity for 90 min. Treatment was repeated every month for two to four cycles. In six patients blood and peritoneal perfusate samples were collected at 0.5, 1, 1.5, 2, 4, 8, 16 and 24 h after drug administration and mitoxantrone was assayed by an HPLC method. RESULTS: Although treatment was generally well tolerated, all patients developed transient intestinal subocclusion. Maximal mitoxantrone plasma concentrations (Cmax), times to Cmax (Tpeak) and area under the curves (AUC) were highly variable between subjects (Cmax 14-337 ng/ml; Tpeak 0.5-8 h; AUC 222-4130 ng x ml(-1) x h). The plasma to peritoneal fluid AUC ratio was significantly higher during the second (0.177) than during the first cycle (0.066), suggesting a cycle-dependent increase in systemic bioavailability. Furthermore, when comparing present data with those reported previously, hyperthermic perfusion may have lowered the mitoxantrone levels in the peritoneal fluid without greatly influencing plasma levels. CONCLUSIONS: Intraperitoneal mitoxantrone administered under hyperthermia to advanced ovarian cancer patients is feasible and well tolerated. Mitoxantrone pharmacokinetics may be altered by repeated intraperitoneal administration (increased bioavailability) and by hyperthermic perfusion (possibly, increased peritoneal tissue uptake).

Renal clearance of N(1)-methylnicotinamide: a sensitive marker of the severity of liver dysfunction in cirrhosis.

BACKGROUND/AIMS: Data have appeared suggesting that an impairment of renal tubular secretion is present in liver cirrhosis, even in the absence of a clinically evident renal dysfunction. To address this question, we evaluated the renal clearance of N(1)-methylnicotinamide (NMN), a marker of the renal secretory function, in healthy subjects and patients with liver cirrhosis of increasing severity, but with a normal glomerular filtration rate. METHODS: The renal clearances of endogenous NMN, inulin, and creatinine were measured in 14 normal subjects and in two groups of age-matched cirrhotic patients (10 Child A and 10 Child C). In 6 subjects, 2 per group, the concentration dependence of the NMN clearance was also studied, following an oral nicotinamide load. RESULTS: Contrary to expectations, the renal NMN clearance increased in cirrhotic patients, in relation to the severity of liver disease (r = 0.83 with Pugh's score; p < 0.001). The NMN-to-inulin clearance ratio increased from a control value of 2.2 +/- (SD) 0.4 to 3.1 +/- 0.2 and 5.2 +/- 0.9 in Child A and Child C cirrhotics, respectively (p < 0.001 for all comparisons), indicating that NMN was completely cleared from plasma in the latter patients. Consistently, the analysis of the concentration dependence of the renal NMN clearance revealed the presence of a carrier-mediated reabsorption which apparently was no longer operating in the decompensated patients. Discriminant analysis showed that renal NMN clearance, and NMN-to-creatinine and NMN-to-inulin clearance ratios could all distinguish between the three study groups, with sensitivities and specificities equal or greater than 90%. CONCLUSIONS: Contrary to previous proposals, NMN is not a probe of general validity for renal tubular secretion. In particular, due to an imbalance between secretion and reabsorption, its renal clearance in liver cirrhosis cannot be used to determine the degree of tubular secretion of which an individual is capable. However, renal NMN clearance appears to be a very sensitive marker of the severity of liver dysfunction in cirrhosis. The potentialities of this renal parameter as a diagnostic and prognostic test in liver cirrhosis deserve further study.

Myocardial region (right or left ventricle) and aetiology of heart failure can influence the inotropic effect of ouabain in failing human myocardium.

AIMS: To investigate whether the inotropic effect of ouabain in failing human myocardium varies according to the heart chamber tested (right or left ventricle) or the aetiology of the heart disease, i.e. ischaemic or idiopathic. METHODS: The inotropic effect of ouabain was measured, as the percentage change in baseline tension, in myocardial strips isolated from right (RV; n=21) and left ventricles (LV; n=21) of hearts explanted from patients with idiopathic (IDC; n=11) and ischaemic cardiomyopathy (CAD; n=10). Concentration-effect curves obtained with ouabain (0.05-1.6 micromol l-1 ) were analysed using the Emax sigmoidal model, and the following parameters were calculated: Emax, EC50, n and EC10 (threshold concentration). The influence of ventricular chamber and heart failure aetiology on these parameters was evaluated by means of a two-way anova. RESULTS: Age and baseline haemodynamic parameters did not differ between IDC and CAD patients. Baseline strip contractility was highly variable (range: 0.48-10.0 mN), but neither ventricular chamber nor aetiology could explain such variability. A two-way anova showed that EC10 was greater in CAD than in IDC preparations (0.097+/-0.013 micromol l-1 vs 0.059+/-0. 009 micromol l-1; 95% C.I. for difference 0.043, 0.071) and Emax was lower in RV than in LV (121+/-21% vs 250+/-38%; 95% C.I. -221, -36), while EC50 and n were not significantly different between groups. CONCLUSIONS: The inotropic effect of ouabain in human myocardium may vary according to aetiology of heart failure and the ventricle being tested. Although our results do not support the hypothesis of increased sensitivity to cardiac glycosides in CAD patients, they may explain the diminished effect observed in patients with RV failure.

Evaluation of measured and calculated creatinine clearances as glomerular filtration markers in different stages of liver cirrhosis.

BACKGROUND: Discrepant results have been published regarding the suitability of creatinine clearance (C(Cr)) as a measure of glomerular filtration rate (GFR) in cirrhotic patients with normal renal function. SUBJECTS AND METHODS: In this study we evaluated the accuracy and precision of measured and calculated C(Cr) as indexes of GFR by comparing their values to those of inulin clearance (C(In)) in 10 healthy subjects and 20 patients with either Child's class A or Child's class C liver cirrhosis. RESULTS: The accuracy and precision of GFR estimates obtained by measuring C(Cr) were good in all three study groups. The mean values of the C(Cr)/C(In) ratio were 1.05, 1.03 and 1.04, respectively, and the corresponding coefficients of variations were 2.9, 2.9 and 3.8%. A close correlation between C(Cr) and C(In) was also found in each study group (r = 0.98, 0.99 and 0.97, respectively, with p < 0.001 in each case). C(Cr) calculated from serum creatinine by means of the Cockcroft-Gault formula (predicted GFR) proved to be a suitable measure of GFR in normal subjects and patients with Child's class A cirrhosis: the predicted-to-true GFR ratios were 0.93 and 0.94, respectively, CV was 12% in both cases. Moreover, a significant correlation between predicted and true GFR was observed in both groups (r = 0.73, p < 0.02 and r = 0.69, p < 0.025, respectively). On the contrary, in Child's class C cirrhotics, calculated C(Cr) significantly overestimated GFR (predicted-to-true GFR ratio 1.23, CV 20%) and no significant correlation was found between predicted and true GFR (r = 0.58, p > 0.05). CONCLUSION: In conclusion, this study shows that measured C(Cr) is a reliable index of GFR in cirrhotic patients, irrespective of the degree of liver dysfunction. Calculated C(Cr) is still an adequate marker of GFR in patients with compensated liver cirrhosis, whereas it overestimates GFR in patients with decompensated cirrhosis. A lower muscle mass, a reduced ability to convert creatine to creatinine, and the presence of ascites are most likely responsible for the overestimation of GFR by the Cockcroft-Gault formula in the latter patients.

Determination of inulin clearance by bolus intravenous injection in healthy subjects and ascitic patients: equivalence of systemic and renal clearances as glomerular filtration markers.

AIMS: Determination of systemic inulin clearance by the standard technique of constant intravenous infusion has long been accepted as a reliable method for measuring glomerular filtration rate (GFR) without urine collection, except in oedematous patients. However, recent studies using standard clearance techniques have claimed that systemic inulin clearance is significantly greater than renal clearance and therefore overestimates GFR. The main purpose of this investigation was to re-evaluate the relationship between systemic and renal inulin clearance using a different technical approach. A reassessment was also made of inulin disposition kinetics. METHODS: Systemic and renal inulin clearances were simultaneously evaluated, in healthy subjects and patients with oedema and ascites, by analysis of the total area under the plasma concentration-time curve (AUC) following bolus intravenous injection. kenal clearance was calculated as the ratio of the total amount recovered in the urine to the AUC, and systemic clearance as dose/AUC. RESULTS: Inulin disposition kinetics were best described by a tri-exponential model. In healthy subjects the volume of the central compartment (mean (s.d.) value 3.86 (1.00) 70 kg(-1)) was slightly greater than the plasma volume; steady-state volume of distribution was 11.00 (1.21) 170 kg(-1), in accordance with the tenet that the inulin space is somewhat smaller than the extracellular fluid volume. The values of systemic and renal inulin clearances were very similar (96.1 (10.0) and 94.6 (12.5) ml min(-1) 70 kg(-1), respectively, in healthy subjects; 104.6 (16.3) and 102.6 (18.5) ml min(-1) in ascitic patients). They were also highly correlated to each other in both healthy subjects (r=0.96, P<0.001) and patients with ascites (r=0.98, P

Effects of cardio-pulmonary bypass on vancomycin plasma concentration decay.

The objective of the study was to investigate possible changes in vancomycin serum levels induced by cardio-pulmonary bypass (CPB). Ten cardiac patients (seven males, three females, aged between 56 and 81), who underwent cardiac surgery requiring CPB, took part in the study. Vancomycin (15 mg kg-1) was intravenously infused over 60 min before anaesthesia and blood samples were taken at appropriate times after drug administration (0, 0.5, 1, 6, 8 h), after starting CPB (0, 5, 30 and 60 min) and after aortic unclamping (0, 5, 30, 60, 120 min). Drug serum concentrations were determined by means of a fluorescence polarization immunoassay. The area under the concentration-time curve (AUC) measured during CPB were compared with the AUC extrapolated in the same interval by fitting a two-compartment pharmacokinetic model to drug concentrations obtained before and after CPB. Five minutes after starting CPB vancomycin serum levels decreased, on average, by 40.9% and remained steadily lower than the expected values over the next 60 min. In the same interval, the measured AUC was 31.7% lower than the expected AUC. In no instance did serum levels fall below the MIC for most common pathogens (1-2 mg l-1). At aortic unclamping serum levels slightly rebounded but tended to remain lower than the expected concentrations over the next 120 min. In conclusion, during CPB vancomycin serum levels invariably decreased but, at the dose employed (15 mg kg-1), remained in a potentially effective range for antimicrobial prophylaxis.

Cardiac effects of quinidine on guinea-pig isolated perfused hearts after in vivo quinidine pretreatment.

1 Experimental and clinical studies suggest that class I and class III antiarrhythmic drugs may be subject to pharmacological tolerance during long term treatment, leading to loss of therapeutic effectiveness. 2 The aim of this study was to ascertain whether prolonged in vivo treatment with the Class Ia agent quinidine can modify cardiac (electrical and mechanical) responses to the drug. 3 A group of guinea-pigs (n = 7) was treated intraperitoneally (q.d.) for 6 days with 75 mg kg-1 quinidine sulphate. Preliminary pharmacokinetic experiments indicated that this dose could attain Plasma concentrations similar to those that are therapeutic in man (2-5 mg l-1). A control group (n = 7) received a saline solution for the same period. 4 Twenty-four hours after the last administration hearts were removed and retrogradely perfused at constant flow (stimulation frequency: 2.5 Hz). The following parameters were measured: maximal derivative of intraventricular pressure (dP/dtmax); coronary perfusion pressure (Cp); PR, QRS and JT intervals, on surface ECG. The effects of quinidine on these parameters were measured at different concentrations (2, 4, 8, 12, 16, 20 microns) and compared in the two experimental groups. 5 In the group quinidine decreased in a dose-dependent manner dP/dt and increased PR and QRS intervals. JT interval was increased at the lowest concentrations and decreased at the highest (biphasic effect). Cp did not change significantly. 6 In the pretreated group quinidine qualitatively produced the same effects on dP/dt and ECG intervals as in control group. Also the magnitude of these effects was not significantly different between the two groups. In contrast with findings in control experiments. Cp was significantly decreased by increasing quinidine concentration. Mean baseline Cp was higher in pretreated than in the control group (though not significantly, P = 0.072) and quinidine addition abolished this difference. Thus, it is suggested that quinidine withdrawal induced a rebound increase in coronary tone, due to the unmasking of vasoconstrictor homeostatic mechanisms elicited by the in vivo vasodilating effect of the drug. 7 In conclusion, our data do not support the possibility that tolerance ensues during long term quinidine treatment, at least as far as electrophysiological and contractility effects are concerned. Further experimental work is needed to explain the appearance of a coronary vasodilating effect in pretreated hearts.