Ramakrishnan Seshadri

Metabolism and elimination of rhodamine 123 in the rat

SummaryLittle is known of the pharmacology of rhodamine 123 (RH-123), an agent reported to have carcinoma-selective experimental antitumor activity. Accordingly, using a high-performance liquid chromatographic assay system with fluorescence detection, we examined the plasma decay and the biliary and urinary elimination of parent drug and metabolites in female Sprague-Dawley rats receiving RH-123 at an intravenous dose (5 mg/kg) equivalent to the therapeutic dose used in murine tumor models. Following drug administration to unconscious animals, plasma levels of drug-associated fluorescence fell in a triphasic manner (t1/2α, 15 min; t1/2β, 1 h; t1/2γ, 4.7 h). In plasma, unchanged drug predominated but lower levels of the deacylated metabolite rhodamine 110 (RH-110) and two unknowns were also detectable throughout the study. Drug fluorescence was recovered extensively in both urine and bile. In unconscious animals with ureteral cannulae, urinary excretion (11.4% of the dose in 6 h) occurred predominantly as unchanged RH-123 (97% of the total), with low levels of RH-110 (2.4%) and two unknowns (<0.6% combined) also being present. Similarly dosed conscious animals (without surgical intervention) housed in metabolic cages showed a comparable pattern of urinary excretion, with 11.9% of the drug dose being recovered in 6 h and 21.9%, by 48 h. Biliary drug elimination accounted for 8% of the delivered dose in 6 h in unconscious animals and for 11% by 36 h in conscious animals fitted with biliary cannulae. In contrast to urinary excretion, in which unchanged drug predominated, only 50% of the fluorescence recovered in bile was attributable to RH-123. The remainder was due to a number of products that were detectable throughout the study. Of these, one present at significant levels was identified as a glucuronide conjugate of RH-123, based on the liberation of parent drug when the purified metabolite was incubated with β-glucuronidase or hydrolyzed with 1 N hydrochloric acid. Further studies with a radiolabeled form of RH-123 are necessary to establish the identity of the remaining unknowns disclosed in this work.

INHIBITION OF MITOCHONDRIAL CARNITINE PALMITOYLTRANSFERASES BY ADRIAMYCIN AND ADRIAMYCIN ANALOGUES

Adriamycin (ADR; doxorubicin) and its highly lipophilic, less toxic analogue N-benzyl-adriamycin-14-valerate (AD 198) were found to inhibit rat heart and liver carnitine palmitoyltransferases of both mitochondrial outer and inner membranes. The outer membrane enzyme was more sensitive to inhibition by these drugs than the inner membrane enzyme, and AD 198 was a more potent inhibitor of these enzymes than ADR. Other analogues of ADR, N-trifluoroacetyladriamycin-14-valerate (AD 32) and N-trifluoroacetyladriamycin-14-O-hemiadipate (AD 143), which are documented as being noncardiotoxic, were also more potent inhibitors of the mitochondrial carnitine palmitoyltransferases than ADR. Overall, the cardiac mitochondrial carnitine palmitoyltransferases seemed to be slightly more sensitive to the inhibitory effects of ADR and its analogues than the liver enzyme. ADR was an uncompetitive inhibitor with respect to palmitoyl-CoA and a noncompetitive inhibitor with respect to carnitine for both mitochondrial outer and inner membrane enzymes. Our data suggest that mitochondria can take up ADR and concentrate it within the matrix, as is known to happen with other positively-charged compounds. More ADR was found associated with the mitochondrial inner membrane than with the outer membrane; this could be due to the greater protein content of the inner membrane rather than drug binding to cardiolipin. Although inhibition of cardiac inner membrane carnitine palmitoyltransferase has been implicated previously as part of the cardiotoxicity mechanism of ADR, the present findings with ADR and its noncardiotoxic analogues do not support this view.

A fluorescence study examining how 14-valerate side chain substitution modulates anthracycline binding to small unilamellar phospholipid vesicles

The intrinsic fluorescence properties of the anthracycline antitumor antibiotics were studied in an effort to understand how 14-valerate side chain substitution modulates drug associations with small unilamellar phospholipid vesicles (SUVs) under near physiological conditions. Drug location and dynamics in fluid-phase dimyristoylphosphatidylcholine (DMPC) bilayers were evaluated for several analogs; accessibilities of bound fluorophores to membrane-impermeable iodide were evaluated in quenching experiments, while the diffusive motions of these agents were studied using lifetime-resolved anisotropy plots. The bulky and hydrophobic valerate substituent was found to further hinder the rotations experienced by a bound drug molecule, with apparent limiting anisotropy (a infinity) values showing increases of 13-82% upon valerate group substitution. In addition, the bimolecular quenching rate constants (unit, 10(9) M-1.s-1) for membrane-bound adriamycin (1.4), N-trifluoroacetyladriamycin (0.4), and their corresponding valerate-substituted analogs (kq values of 1.1 and 0.5, respectively) reveal that the side chain is a weak modulator of fluorophore penetration into the bilayer, with stronger modulation being achieved through amino group substitution. Similar results were obtained for drugs bound to negatively-charged dimyristoylphosphatidylglycerol (DMPG) bilayers. Finally, comparison of the equilibrium binding affinities of the various congeners for electroneutral DMPC versus negatively-charged DMPG bilayers demonstrate that positively-charged parent anthracyclines display high levels of selective binding to negatively-charged phospholipids, unlike valerate-substituted analogs which display no such selectivity. The modulation of anthracycline-membrane interactions achieved through valerate substitution offers potential explanations, at least in part, for some of the novel biological properties of valerate-containing anthracyclines.

Interaction of N-alkylanthracyclines with lipid bilayers: correlations between partition coefficients, lipid phase distributions and thermotropic behavior

The thermotropic behavior of multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC), or of DPPC in admixture with cardiolipin or cholesterol, in the presence of various N-alkyl derivatives of both adriamycin and adriamycin-14-valerate has been investigated by high sensitivity differential scanning calorimetry. The analogues, particularly the 14-valerate derivatives, which were most lipophilic as judged by their lipid/buffer, and to a lesser extent by their octanol/buffer, partition coefficients, were the most effective in depressing the tm of the investigated lipids; correlations, however, were not absolute. Other factors, such as the distribution of the drugs between the solid and liquid-crystalline phases of the bilayer, were also important to the observed membrane perturbations. With all anthracyclines, however, no major changes in the transition enthalpy were observed. In the case of vesicles prepared from pure DPPC, curve fitting analysis based on ideal solution theory (J.M. Sturtevant (1984) Proc. Natl. Acad. Sci. USA 81, 1398-1400) applied at relatively low drug concentrations where single peak transitions were produced, adequately described the differential scanning calorimetric results. At high drug concentrations, however, the presence of multi-peak transitions were indicative of non-ideality.

Comparative uptake and retention of adriamycin andN-benzyladriamycin-14-valerate in human CEM leukemic lymphocyte cell cultures

SummaryN-Benzyladriamycin-14-valerate (AD 198) is a new lipophilic adriamycin (ADR) analogue that shows marked therapeutic superiority to ADR in murine tumor model systems yet differs mechanistically from ADR in a number of ways. Among its other properties, AD 198 produces a delayed but profound effect on cell-cycle progression and a pattern of continuing DNA damage in cultured cells briefly exposed to the drug. Using radiolabeled drug forms and radioassays combined with HPLC separation and fluorimetric detection techniques, aspects of drug accumulation, biotransformation, and retention in cultured human CEM leukemic lymphocytes were studied, in part to determine a possible pharmacologic basis for the latent effects seen with this drug. In addition, the cellular pharmacology of AD 198 and ADR were comparatively examined under identical experimental conditions. When CEM cells were incubated with drug at equi-growth inhibitory/minimally cytotoxic concentrations (AD 198, 1.0 μM; ADR, 0.1 μM), a number of differences were apparent. Under conditions of continuous 24-h drug exposure, a slow cellular accumulation and equilibration was observed with ADR (cell: medium equilibrium, 1:11 after 4–6 h), whereas the uptake of AD 198 was rapid and extensive (cell: medium equilibrium, 3:1 within 30 min). In drug-retention studies, when cells were pretreated at the same drug concentrations as before (AD 198 for 1 h; ADR for 4 h) and then transferred to drug-free media, both compounds re-equilibrated their intracellular drug content with the fresh media, losing about 50% of their respective anthracycline levels. Liquid chromatographic analysis of ADR-treated cultures under both sets of conditions showed the parent drug to be the only intracellular anthracycline species, whereas analysis of AD 198-treated cultures revealed two fluorescent signals corresponding to the parent drug and its 14-deesterified biotransformation product,N-benzyladriamycin (AD 288). Levels of AD 288 rose from 2% of the total intracellular anthracycline content immediately on drug admixture to 61% following 24 h continuous drug exposure and to 69% at 24 h in cells exposed to drug for 1 h and then continued in drug-free media for 24 h. At all times, the balance of the intracellular anthracycline fluorescence was attributable to the parent drug; no ADR was detectable in AD 198-treated cells by either fluorescence detection or radioassay. Thus, AD 198 is not a prodrug form of ADR, and the in vitro effects of this agent, including the latent effects on cell-cycle inhibition and DNA damage seen in cells following short-term drug exposure, can be explained on the basis of the high levels of active parent drug and biotransformation product that accumulate and persist in the cells.

Pharmacology of N-benzyladriamycin-14-valerate in the rat

Purpose: N-Benzyladriamycin-14-valerate (AD 198) is a semisynthetic anthracycline analogue superior to doxorubicin (DOX) both in vitro and in experimental rodent tumor models, and with differing mechanistic properties from those of the parental antibiotic agent. In the present study, we examined the metabolic fate and hematotoxicity of AD 198 in rats, with a view to determining whether some of the therapeutic properties observed for this drug might be due to a DOX prodrug effect. Methods: Samples of plasma, bile and urine were obtained at various times following intravenous (i.v.) [14C]–AD 198 administration to rats and were analyzed by reversed-phase HPLC with flow–fluorescence detection and complementary liquid scintillography. In other animals, red blood cell and white blood cell (WBC) counts were determined for blood obtained by retrobulbar sampling on selected days from groups of animals receiving either AD 198 or DOX at several dose levels, as well as from vehicle controls. Results: Following a single iv dose of [14C]-AD 198 (5 mg/kg; equivalent to the optimal murine antitumor dose) in anesthetized rats, a triphasic plasma decay pattern for parental drug was evident with extremely rapid α and β phases followed by a very long terminal elimination phase. Principal plasma products included N–benzyladriamycin (AD 288) and N–benzyladriamycinol (AD 298) together with very low levels of DOX and doxorubicinol (DOXOL). Analysis of bile from anesthetized and conscious animals receiving AD 198 revealed DOX to be the principal biliary fluorescent species together with low levels of AD 288, AD 298 and DOXOL; no parental drug was seen. By contrast, AD 288 was the principal urinary product, together with low levels of AD 298 and DOX; again, no parental drug was evident. Dose recovery (8 h) in the respective bile and urine of anesthetized rats was 12.4% and 13.2% based upon total fluorescence versus 1% and 15.3% of the administered radiolabel. In conscious animals, 13.4% of drug fluorescence was recovered in the bile (48 h), while in urine 16.6% and 77.1% of drug fluorescence and radiolabel, respectively, were eliminated over 72 h. The discrepancy between recovery of drug fluorescence and 14C was due to the production of nonfluorescent hippuric acid (benzoylglycine) and N–benzyl daunosamine as a consequence of hepatic and renal drug metabolism. In the separate hematotoxicity studies, AD 198 (24.6 mg/kg i.v.; equivalent to the murine LD50 dose), produced a 45% reduction (nadir day 3–5) in WBC count, with recovery by day 10. By contrast, DOX (10 mg/kg i.v.; equivalent to the mouse highest nonlethal dose) produced an 80% decline in WBC with only partial recovery by day 17. Conclusions: By virtue of the low systemic DOX levels and low hematotoxicity observed in rats receiving AD 198, the in vivo therapeutic superiority of AD 198 cannot be attributed to substantial intracellular DOX generation. The conclusion that the therapeutic superiority of AD 198 compared to DOX results from the mechanistic differences between these two drugs is further supported by recent observations on their biochemical differences with regard to protein kinase C and topoisomerase II inhibition.

An Analytical System for the Detection and Quantitation of Rhodamine-123 in Biological Samples

Abstract Rhodamine-123, a mitochondrial-specific dye, is currently of interest as a potential anticarcinoma agent. We now report an analytical method suitable for monitoring this agent in a variety of biological matrices. The drug is easily extractable into organic solvent in high yield (>90%). Analysis is conducted by reversed-phase HPLC using a phenyl column and an organic/aqueous buffer mobile phase, with signal detection by flow fluorometry. The assay shows linearity over the concentration range of 5-4000 ng/ml, with a limit of sensitivity of 100 pg. Preliminary animal data confirms the suitability of the assay for detailed drug metabolism and dispositional studies.

Pharmacology of N,N-di(n-butyl)adriamycin-14-valerate in the rat.

Lipophilic N-alkylanthracyclines such as AD 198 (N-benzyladriamycin-14-valerate) or AD 201 [N,N-di-(n-propyl)adriamycin-14-valerate], which exert their cytotoxicity through mechanisms which are not yet fully defined, possess inherent abilities to circumvent multidrug resistance in vitro and in vivo, possibly through alterations in normal intracellular drug trafficking. As part of structure-activity studies with this class of agent, we have now examined the pharmacology of AD 202 [N,N-di(n-butyl)adriamycin-14-valerate], another analog possessing superior antitumor activity to doxorubicin in vivo and an ability to circumvent multidrug resistance in vitro. Following the administration of AD 202 (20 mg/kg, i.v.) to anesthetized rats, rapid drug distribution (T1/2 5 min) was followed by more gradual elimination (T1/2 3.6 h). Plasma clearance of AD 202 (224 ± 63.6 ml/minper kg) and steady state volume of distribution (25.7 ± 11.1 I/kg) were indicative of extensive tissue sequestration and/or a large degree of extra-hepatic metabolism. The parent drug predominated in plasma until 20 min, thereafter N,N-di(n-butyl)adriamycin became the principal circulating anthracycline. The systemic exposure to this biotransformation product (area under the plasma concentration-time curve from time zero to 480 min AUC0-480 281672 ng-min/ml) was > tenfold higher than for the other detected plasma products (N-butyladriamycin-14-valerate, N-butyladriamycin, and three unidentified fluorescent signals; PI-3). Total urinary elimination over 8 h was limited (1.9% of dose), occurring predominantly as N,N-di(n-butyl)adriamycin (1.2% of dose), N-butyladriamycin (0.4% of dose), and their corresponding 13-carbinol metabolites (< 0.1% of dose each). Low levels of adriamycin (ADR), aglycones and two unidentified products were also seen. Parental AD 202 was found in urine only up to 1 h. By contrast, hepatic elimination of parent drug was seen, albeit at low levels, through 8 h. Excretion by this route (22% of dose) occurred principally as N-butyladriamycin (8% of dose), N-butyladriamycinol (2.1% of dose) with lower levels of N,N-di(n-butyl)adriamycin (1.6% of dose), N,N-di(n-butyl)adriamycin (0.8% of dose), and aglycones (4.3% of dose, combined). Other products included ADR (1.1% of dose) and two unidentified signals (3.4% of dose, combined). The relatively poor mass balance in these studies is attributed to prolonged intracellular retention (elimination T1/2 24.2 h) of N,N-di(n-butyl)adriamycin. Thus, in common with other N-alkylanthracyclines, the pharmacology of AD 202 is complex but its therapeutic properties clearly are not derived from an ADR prodrug effect. Significant differences continue to be noted as to the metabolic fate of congeners of this class of anthracycline analogs.