Recent advances in prodrugs as drug delivery systems.

Prodrugs are a class of drug derivatives with little or no pharmacological activity that are converted in vivo to therapeutically active compounds. The primary utility of a prodrug approach is to improve pharmaceutical properties. Because it does not alter the primary structure of the parent drug, the synthesis of prodrugs is usually much less difficult than the synthesis of analogs. The derived physicochemical properties of the resulting derivatives can be carefully tailored by means of structural modification of the promoiety. However, sufficient levels of intrinsic activity of the parent drug need to be assured through in vivo cleavage of the prodrug. The prodrug approach has been successfully applied to a wide variety of drugs. This article briefly discusses advances in strategies for development of prodrugs and their mechanisms of drug release.

The effects of chiral isolates of methadone on the cardiac potassium channel IKr.

OBJECTIVES: Methadone is a synthetic opioid, an analgesic and an antiaddictive. QT prolongation as well as torsade de pointes ventricular tachycardia and death have been reported with methadone. Methadone's proarrhythmic toxicity is related to the inhibition of cardiac IKr channel and prolongation of the action potential. We hypothesized that the 2 isomers of methadone may have different effects on the IKr channel. METHODS: The effects of the isomers on IKr were evaluated by using an oocytes system with heterogeneously expressed human ether-a-go-go-related gene (HERG) using the 2 electrode voltage clamp technique. r- and s-methadone were obtained by employing chiral high-performance liquid chromatography, separating methadone into 2 isolates, with optical rotations of -141 and +143 degrees. RESULTS: At concentrations of 0.01, 0.03, 0.1, 1 and 3 mM, r/s-methadone produced a dose-dependent inhibition of HERG by 17 +/- 5, 23 +/- 4, 40 +/- 4, 57 +/- 3, 69 +/- 3 and 80 +/- 1%, respectively. The IC50 of r/s-methadone was 0.21 +/- 0.02 mM. At 0.1, 0.3 and 1 mM, s-methadone reduced HERG current by 50 +/- 4, 76 +/- 5 and 87 +/- 5%, respectively, while r-methadone reduced HERG current by 26 +/- 4, 53 +/- 3 and 77 +/- 3%, respectively. CONCLUSIONS: There was a significant difference (p < 0.01) in the percentage of current inhibition between r- and s-methadone, at 0.1 and 0.3 mM (52% reduction). Thus, r-methadone may be a safer agent due to less QT effect.

The additive effects of the active component of grapefruit juice (naringenin) and antiarrhythmic drugs on HERG inhibition.

BACKGROUND: Grapefruit juice causes significant QT prolongation in healthy volunteers and naringenin has been identified as the most potent human ether-a-go-go-related gene (HERG) channel blocker among several dietary flavonoids. The interaction between naringenin and I(Kr)-blocking antiarrhythmic drugs has not been studied. We evaluated the effect of combining naringenin with I(Kr)-inhibiting antiarrhythmic drugs on cardiac I(Kr). METHODS AND RESULTS: I(Kr) current was studied by using HERG expressed in Xenopus oocytes, and the two-electrode voltage clamp technique was employed. Antiarrhythmic drugs (azimilide, amiodarone, dofetilide and quinidine) were tested. Experiments were performed at room temperature. Naringenin blocked HERG current dose dependently with an IC(50) of 173.3 +/- 3.1 microM. Naringenin 100 microM alone inhibited HERG current by 31 +/- 6%, and this inhibitory effect was increased with coadministration of 1 or 10 microM antiarrhythmic drugs. When 100 microM naringenin was added to antiarrhythmic drugs, greater HERG inhibition was demonstrated, compared to the current inhibition caused by antiarrhythmic drugs alone. Addition of naringenin significantly increased current inhibition (p < 0.05). CONCLUSIONS: There is an additive inhibitory effect on HERG current when naringenin is combined with I(Kr)-blocking antiarrhythmic drugs. This additive HERG inhibition could pose an increased risk of arrhythmias by increasing repolarization delay and possible repolarization heterogeneity.

Extracellular acidification and hyperkalemia induce changes in HERG inhibition by ibutilide.

BACKGROUND: A high incidence of proarrhythmia has been reported with ibutilide, especially in patients with underlying heart diseases. Our previous studies have shown that extracellular acidosis and hyperkalemia attenuate the HERG-inhibitory effect of proarrhythmic drugs, e.g. quinidine, but have little impact on the less-proarrhythmic drug amiodarone. We hypothesized that ibutilide would behave like quinidine in the presence of extracellular acidosis and hyperkalemia. METHODS AND RESULTS: HERG was expressed on Xenopus oocytes, and the two-electrode voltage clamp technique was employed. Our results showed that ibutilide was a potent HERG inhibitor. When extracellular solution contained 5 mM KCl and pH was 7.4, the IC(50) of ibutilide was 0.9 +/- 0.1 microM. The inhibitory effect of ibutilide was attenuated when extracellular pH decreased to 6.2. There was a significant difference in current inhibition by ibutilide at pH 7.4 versus pH 6.2 (p < 0.01). When the extracellular potassium concentration was increased from 5 to 10 mM, ibutilide produced less current inhibition, and the IC(50) was increased to 2.0 +/- 0.1 microM. CONCLUSION: Extracellular acidosis and hyperkalemia attenuate the HERG-inhibitory effect of ibutilide. The differences in HERG inhibition between acidic and hyperkalemic regions compared to normal regions in the myocardium may result in heterogeneity in repolarization, which may contribute to the proarrhythmic toxicity of ibutilide.

The effects of quinidine and its chiral isolates on erg-1sm potassium current and correlation with gastrointestinal augmentation.

Smooth-muscle erg 1 (erg1-sm) potassium channel has been recently reported to participate in the modulation of gastrointestinal contractility. Because quinidine inhibits cardiac potassium channel and as a result augments gastrointestinal contractility, it was thought that quinidine may affect erg1-sm. Studies were undertaken to evaluate the effects of quinidine and its chiral isolates on gastrointestinal erg1-sm potassium current and correlate these effects with colon contractility. Chiral separation (high-performance liquid chromatography technique), mass spectrometry, and optical rotation determination were performed to obtain chiral isolates needed for experiments. The erg1-sm potassium channel was expressed in Xenopus oocytes, and the two-electrode patch clamp technique was employed for recording. An isolated rat colon preparation was employed to measure changes in contractility. As a result of chiral separation, two peaks were obtained with elution times of 8.31 and 8.66 minutes, both with a molecular weight of 324; the optical rotations of racemate isolates X and Y were: +258 degrees, +/-0 degrees; and +217 degrees, respectively. The percentage changes in amplitudes of colon contraction (from baseline) were determined at different concentrations of quinidine and for the two isolates in five experiments in each group. Quinidine 0.1, 1, and 10 microM increased contractility by 79 +/- 34, 125 +/- 42, and 217 +/- 51 (P < or = 0.05); for isolate X, the values were 70 +/- 20, 115 +/- 32, and 272 +/- 32 (P < or = 0.05), and for isolate Y the values were 22 +/- 12, 46 +/- 17, and 59 +/- 22. The inhibition of erg1-sm currents by quinidine was 19 +/- 4, 21 +/- 5, and 48 +/- 6 (P < or = 0.05), respectively; that by isolate X was 20 +/- 4, 23 +/- 5, and 39 +/- 7 (P < or = 0.05), and that by isolate Y was 22 +/- 4, 21 +/- 4, and 31 +/- 6. One chiral isolate and quinidine markedly augmented contractility, whereas quinidine and the two chiral isolates inhibited the erg1-sm potassium currents to a similar extent. These results suggest that erg1-sm inhibition does not explain gastrointestinal contractile augmentation caused by the quinidine racemate and its chiral isolates.

The effect of high extracellular potassium on IKr inhibition by anti-arrhythmic agents.

BACKGROUND: Hyperkalemia is a potentially life-threatening disorder frequently occurring in hospitalized patients. The ischemic myocardium releases potassium into the extracellular space which can cause regional hyperkalemia. These changes may modify the effects of anti-arrhythmic drugs acting on the rapid component of the delayed rectifier potassium current (IKr). We evaluated the influence of increased extracellular potassium concentration [K(+)](e) on IKr inhibition by amiodarone, azimilide, dofetilide, quinidine and sotalol. METHODS AND RESULTS: Experiments were performed at room temperature. IKr current was studied by using HERG gene expressed in Xenopus oocytes as a model of cardiac IKr. Two-electrode voltage clamp technique was employed. The recording bath solutions contained either 5 or 10 mmol/l KCl. Amiodarone, azimilide, dofetilide, quinidine and sotalol all produced a dose-dependent inhibition of HERG current. At 5 mmol/l [K(+)](e), the IC(50) was 37.0 +/- 12.5 microM for amiodarone, 5.8 +/- 0.4 microM for azimilide, 1.5 +/- 0. 2 microM for dofetilide, 9.1 +/- 1.5 microM for quinidine, and 5.1 +/- 0.8 mM for sotalol. Raising the extracellular potassium to 10 mmol/l, HERG block by azimilide, dofetilide, quinidine and sotalol was significantly decreased, while the block by amiodarone was unchanged. The differences in the percentage current block produced by 3 microM drugs at 5 and 10 mmol/l [K(+)](e) were: -0.9% for amiodarone, 13.8% for quinidine, 20.5% for azimilide, and 16.2% for dofetilide. The differences in percentage block between 5 and 10 mmol/l [K(+)](e) by sotalol 10 and 30 mM were 7.1 and 5.6%. At 10 mmol/l [K(+)](e), the IC(50) was increased for azimilide, dofetilide, quinidine and sotalol but not for amiodarone; the IC(50) was 24.7 +/- 7.4 microM for amiodarone, 29.3 +/- 3.9 microM for azimilide, 2.7 +/- 0.2 microM for dofetilide, 27.6 +/- 4.0 microM for quinidine, and 7.2 +/- 1.7 mM for sotalol. CONCLUSION: Inhibition of IKr by azimilide, quinidine, dofetilide and sotalol was diminished by increasing [K(+)](e), while the inhibition by amiodarone was unchanged at normal and high [K(+)](e). The differential effects of azimilide, dofetilide, quinidine and sotalol at normal and high [K(+)](e) could be pro-arrhythmic by favoring re-entry arrhythmias. These results further support the unique electrophysiological effect of amiodarone.

The differential antibacterial and gastrointestinal effects of erythromycin and its chiral isolates.

The use of erythromycin has been limited by the gastrointestinal side effect properties, which include abdominal distress and diarrhea. To evaluate the possibility of reducing the toxicity of erythromycin, studies were undertaken to separate erythromycin into chiral isolates and then to test the activity of these chiral isolates on gastrointestinal contractility and bacteriostatic actions. Gastrointestinal contractility was obtained by the use of isolated strips of a rat colon. Antibacterial activity was used by obtaining the MICs of erythromycin and isolated agents against Enterococcus faecalis ATCC 29212. ANOVA was performed using the SPSS v.10 to determine statistical differences in the MICs and the amplitude and frequency of spike bursts. Results were expressed as mean+/-SE (N=5). The MICs (microg/mL) of erythromycin (racemate), chiral isolate X, and chiral isolate Y were 0.45+/-0.29, 0.53+/-0.24 (n.s.), and 0.2+/-0.07 (P

A mechanism for the potential proarrhythmic effect of acidosis, bradycardia, and hypokalemia on the blockade of human ether-a-go-go-related gene (HERG) channels.

Many drugs are proarrhythmic by inhibiting the cardiac rapid delayed rectifier potassium channel (IKr). In this study, we use quinidine as an example of highly proarrhythmic agent to investigate the risk factors that may facilitate the proarrhythmic effects of drugs. We studied the influence of pacing, extracellular potassium, and pH on quinidine's IKr blocking effect, all potential factors influencing quinidine's cardiac toxicity. Since the HERG gene encodes IKr, we studied quinidine's effect on HERG expressed in Xenopus oocytes by the 2-electrode voltage clamp technique. When extracellular K+ was 5 mmol/L, quinidine blocked the HERG current dose dependently, with an IC50 of 6.3 +/- 0.2 micromol/L. The blockade was much more prominent at more positive membrane potentials. The inhibition of HERG by quinidine was not use dependent. There was no significant difference between block with or without pacing. When extracellular K+ was lowered to 2.5 mmol/L, the current inhibition by quinidine was enhanced, and IC50 decreased to 4.6 +/- 0.5 micromol/L. At 10 mmol/L extracellular K+, there was less inhibition by quinidine and the IC50 was 11.2 +/- 3.1 micromol/L. Extracellular acidification decreased both steady state and tail currents of HERG. We conclude that the inhibitory effect of quinidine on IKr was decreased with extracellular acidification, which may produce heterogeneity in the repolarization between normal and ischemic cardiac tissue. Thus, the use-independent blockade of IKr by QT-prolonging agents such as quinidine may contribute to cardiac toxicity with bradycardia, hypokalemia, and acidosis further exaggerating the proarrhythmic potential of these agents.

The influence of extracellular acidosis on the effect of IKr blockers.

BACKGROUND: Myocardial infarction causes the acidification of the cellular environment and the resultant acidosis maybe arrhythmogenic. The effect of acidosis on the action of antiarrhythmic drugs, an important issue in the antiarrhythmic drug therapy after myocardial infarction, remains to be studied. METHODS: To evaluate the effect of acidosis on rectifier potassium current (Ikr) blockers, the human ether-a-go-go-related gene (HERG), which encodes IKr, was expressed in Xenopus laevis oocytes. The two electrodes voltage clamp technique was used and the experiments were performed at room temperature. RESULTS: Quinidine (10 microM) inhibited HERG tail current by 37% +/- 5% at pH7.4. The block decreased to 5% +/- 2% with extracellular pH at 6.2. Dofetilide (0.3 microM) inhibited HERG tail current by 34% +/- 3% and 1% +/- 2% at extracellular pH 7.4 and 6.2, respectively. Azimilide (10 microM) inhibited HERG tail current by 59% +/- 3% and 17% +/- 3% at extracellular pH 7.4 and 6.2. There were significant differences in the HERG inhibition by quinidine, dofetilide, and azimilide between pH 7.4 and pH 6.2 (P < .01). The drug concentration blocking 50% of current (IC50) was 5.8 +/- 0.3 microM for azimilide, 9.9 +/- 1.0 microM for quinidine, and 0.5 +/- 0.02 microM for dofetilide at pH 7.4. When extracellular pH was decreased from 7.4 to 6.2, the IC50 increased to 95.5 +/- 11.3 microM for azimilide, 203.2 +/- 15.7 microM for quinidine, and 12.6 +/- 1.2 microM for dofetilide. Unlike quinidine, dofetilide, and azimilide, there was no significant difference in the percentage of current block by amiodarone between pH 6.2 and 7.4. For amiodarone, the IC50 was 38.3 +/- 8.5 microM at pH 7.4 and 27.3 +/- 1.6 microM at pH 6.2. CONCLUSION: Our data show that the Ikr blocking effect of azimilide, dofetilide, and quinidine was attenuated at acid pH, whereas this was not the case for amiodarone. These observations may explain the efficacy of amiodarone in reducing arrhythmic death in patients after a myocardial infarction compared with other IKr blockers.