Effects of the ester moiety on stereoselective hydrolysis of several propranolol prodrugs in rat tissues

ET-CORM Mediated Vasorelaxation of Small Mesenteric Arteries: Involvement of Kv7 Potassium Channels

Although the vasoactive properties of carbon monoxide (CO) have been extensively studied, the mechanism by which CO mediates vasodilation is not completely understood. Through-out published studies on CO mediated vasodilation there is inconsistency on the type of K⁺-channels that are activated by CO releasing molecules (CORMs). Since the vasorelaxation properties of enzyme triggered CORMs (ET-CORMs) have not been studied thus far, we first assessed if ET-CORMs can mediate vasodilation of small mesenteric arteries and subsequently addressed the role of soluble guanylate cyclase (sGC) and that of K-channels herein. To this end, 3 different types of ET-CORMs that either contain acetate (rac-1 and rac-4) or pivalate (rac-8) as ester functionality, were tested ex vivo on methoxamine pre-contracted small rat mesenteric arteries in a myograph setting. Pre-contracted mesenteric arteries strongly dilated upon treatment with both types of acetate containing ET-CORMs (rac-1 and rac-4), while treatment with the pivalate containing ET-CORM (rac-8) resulted in no vasodilation. Pre-treatment of mesenteric arteries with the sGC inhibitor ODQ abolished rac-4 mediated vasodilation, similar as for the known sGC activator SNP. Likewise, rac-4 mediated vasodilation did not occur in KCL pretreated mesenteric arteries. Although mesenteric arteries abundantly expressed a variety of K⁺-channels only Kv7 channels were found to be of functional relevance for rac-4 mediated vasodilation. In conclusion the current results identified Kv7 channels as the main channel by which rac-4 mediates vasodilation. In keeping with the central role of Kv7 in the control of vascular tone and peripheral resistance these promising ex-vivo data warrant further in vivo studies, particularly in models of primary hypertension or cardiac diseases, to assess the potential use of ET-CORMs in these diseases.

N-octanoyl dopamine treatment of endothelial cells induces the unfolded protein response and results in hypometabolism and tolerance to hypothermia

Aim

N-acyl dopamines (NADD) are gaining attention in the field of inflammatory and neurological disorders. Due to their hydrophobicity, NADD may have access to the endoplasmic reticulum (ER). We therefore investigated if NADD induce the unfolded protein response (UPR) and if this in turn influences cell behaviour.

Methods

Genome wide gene expression profiling, confirmatory qPCR and reporter assays were employed on human umbilical vein endothelial cells (HUVEC) to validate induction of UPR target genes and UPR sensor activation by N-octanoyl dopamine (NOD). Intracellular ATP, apoptosis and induction of thermotolerance were used as functional parameters to assess adaptation of HUVEC.

Results

NOD, but not dopamine dose dependently induces the UPR. This was also found for other synthetic NADD. Induction of the UPR was dependent on the redox activity of NADD and was not caused by selective activation of a particular UPR sensor. UPR induction did not result in cell apoptosis, yet NOD strongly impaired cell proliferation by attenuation of cells in the S-G2/M phase. Long-term treatment of HUVEC with low NOD concentration showed decreased intracellular ATP concentration paralleled with activation of AMPK. These cells were significantly more resistant to cold inflicted injury.

Conclusions

We provide for the first time evidence that NADD induce the UPR in vitro. It remains to be assessed if UPR induction is causally associated with hypometabolism and thermotolerance. Further pharmacokinetic studies are warranted to address if the NADD concentrations used in vitro can be obtained in vivo and if this in turn shows therapeutic efficacy.

Different design of enzyme-triggered CO-releasing molecules (ET-CORMs) reveals quantitative differences in biological activities in terms of toxicity and inflammation

Acyloxydiene–Fe(CO)₃ complexes can act as enzyme-triggered CO-releasing molecules (ET-CORMs). Their biological activity strongly depends on the mother compound from which they are derived, i.e. cyclohexenone or cyclohexanedione, and on the position of the ester functionality they harbour. The present study addresses if the latter characteristic affects CO release, if cytotoxicity of ET-CORMs is mediated through iron release or inhibition of cell respiration and to what extent cyclohexenone and cyclohexanedione derived ET-CORMs differ in their ability to counteract TNF-α mediated inflammation. Irrespective of the formulation (DMSO or cyclodextrin), toxicity in HUVEC was significantly higher for ET-CORMs bearing the ester functionality at the outer (rac-4), as compared to the inner (rac-1) position of the cyclohexenone moiety. This was paralleled by an increased CO release from the former ET-CORM. Toxicity was not mediated via iron as EC₅₀ values for rac-4 were significantly lower than for FeCl₂ or FeCl₃ and were not influenced by iron chelation. ATP depletion preceded toxicity suggesting impaired cell respiration as putative cause for cell death. In long-term HUVEC cultures inhibition of VCAM-1 expression by rac-1 waned in time, while for the cyclohexanedione derived rac-8 inhibition seems to increase. NFκB was inhibited by both rac-1 and rac-8 independent of IκBα degradation. Both ET-CORMs activated Nrf-2 and consequently induced the expression of HO-1.

This study further provides a rational framework for designing acyloxydiene–Fe(CO)₃ complexes as ET-CORMs with differential CO release and biological activities. We also provide a better understanding of how these complexes affect cell-biology in mechanistic terms.

Enzyme-triggered CO-releasing molecules (ET-CORMs): evaluation of biological activity in relation to their structure

Acyloxydiene-Fe(CO)3 complexes act as enzyme-triggered CO-releasing molecules (ET-CORMs) and can deliver CO intracellularly via esterase-mediated hydrolysis. The protective properties of structurally different ET-CORMs on hypothermic preservation damage and their ability to inhibit VCAM-1 expression were tested on cultured human umbilical vein endothelial cells (HUVEC) and renal proximal tubular epithelial cells (PTEC) using a structure-activity approach. Cytotoxicity of ET-CORMs, protection against hypothermic preservation damage, and inhibition of VCAM-1 expression were assessed. Cytotoxicity of 2-cyclohexenone and 1,3-cyclohexanedione-derived ET-CORMs was more pronounced in HUVEC compared to PTEC and was dependent on the position and type of the ester (acyloxy) substituent(s) (acetate>pivalate>palmitate). Protection against hypothermic preservation injury was only observed for 2-cyclohexenone-derived ET-CORMs and was not mediated by the ET-CORM decomposition product 2-cyclohexenone itself. Structural requirements for protection by these ET-CORMs were different for HUVEC and PTEC. Protection was affected by the nature of the ester functionality in both cell lines. VCAM-1 expression was inhibited by both 2-cyclohexenone- and 1,3-cyclohexanedione-derived ET-CORMs. 2-Cyclohexenone, but not 1,3-cyclohexanedione, also inhibited VCAM-1 expression. We demonstrate that structural alterations of ET-CORMs significantly affect their biological activity. Our data also indicate that different ET-CORMs behave differently in various cell types (epithelial vs endothelial). These findings warrant further studies not only to elucidate the structure-activity relation of ET-CORMs in mechanistic terms but also to assess if structural optimization will yield ET-CORMs with restricted cell specificity.

Retooling manganese(III) porphyrin-based peroxynitrite decomposition catalysts for selectivity and oral activity: a potential new strategy for treating chronic pain

Redox-active metalloporphyrins represent the most well-characterized class of catalysts capable of attenuating oxidative stress in vivo through the direct interception and decomposition of superoxide and peroxynitrite. While many interesting pharmacological probes have emerged from these studies, few catalysts have been developed with pharmaceutical properties in mind. Herein, we describe our efforts to identify new Mn(III)-porphyrin systems with enhanced membrane solubilizing properties. To this end, seven new Mn(III)-tetracyclohexenylporphyin (TCHP) analogues, 7, 10, 12, 15, and 16a-c, have been prepared in which the beta-fused cyclohexenyl rings provide a means to shield the charged metal center from the membrane during passive transport. Compounds 7, 15, and 16a-c have been shown to be orally active and potent analgesics in a model of carrageenan-induced thermal hyperalgesia. In addition, oral administration of compound 7 (10-100 mg/kg, n=5) has been shown to dose dependently reverse mechano-allodynia in the CCI model of chronic neuropathic pain.

Presystemic metabolism and intestinal absorption of antipsoriatic fumaric acid esters

Psoriasis is a chronic inflammatory skin disease. Its treatment is based on the inhibition of proliferation of epidermal cells and interference in the inflammatory process. A new systemic antipsoriasis drug, which consists of dimethylfumarate and ethylhydrogenfumarate in the form of their calcium, magnesium and zinc salts has been introduced in Europe with successful results. In the present study, a homologous series of mono- and diesters of fumaric acid has been studied with respect to the sites and kinetics of presystemic ester degradation using pancreas extract, intestinal perfusate, intestinal homogenate and liver S9 fraction. In addition, intestinal permeability has been determined using isolated intestinal mucosa as well as Caco-2 cell monolayers, in order to obtain estimates of the fraction of the dose absorbed for these compounds. Relationships between the physicochemical properties of the fumaric acid esters and their biological responses were investigated. The uncharged diester dimethylfumarate displayed a high presystemic metabolic lability in all metabolism models. It also showed the highest permeability in the Caco-2 cell model. However, in permeation experiments with intestinal mucosa in Ussing-type chambers, no undegraded DMF was found on the receiver side, indicating complete metabolism in the intestinal tissue. The intestinal permeability of the monoesters methyl hydrogen fumarate, ethyl hydrogen fumarate, n-propylhydrogen fumarate and n-pentyl hydrogen fumarate increased with an increase in their lipophilicity, however, their presystemic metabolism rates likewise increased with increasing ester chain length. It is concluded that for fumarates, an increase in intestinal permeability of the more lipophilic derivatives is counterbalanced by an increase in first-pass extraction.

In vitro stability and in vivo pharmacokinetic studies of a model opioid peptide, H-Tyr-D-Ala-Gly-Phe-D-Leu-OH (DADLE), and its cyclic prodrugs

In vitro stability and in vivo pharmacokinetic studies of a model opioid peptide, H-Tyr-D-Ala-Gly-Phe-D-Leu-OH (DADLE), and its cyclic prodrugs (acyloxyalkoxy-based cyclic prodrug of DADLE, coumarinic acid-based cyclic prodrug of DADLE, and oxymethyl-modified coumarinic acid-based cyclic prodrug of DADLE) were conducted. The enzymatic stability of DADLE and its prodrugs in various biological media was determined at 37 degrees C in the presence and absence of paraoxon, a known esterase inhibitor. The prodrugs exhibited metabolic stability to exo- and endopeptidases, and esterase-catalyzed bioconversion of the prodrugs to DADLE was observed. For pharmacokinetic studies in rats, various biological samples (blood, bile, urine, and brain) were collected after i.v. administration of DADLE and its prodrugs. The samples were analyzed by high-performance liquid chromatography with tandem mass spectrometric detection, and the conversion from the prodrugs to intermediates to DADLE was monitored. The prodrugs exhibited similar pharmacokinetic properties and showed improved stability compared with DADLE in rat blood. This increased stability led to higher plasma concentrations of DADLE after i.v. administration of the prodrugs compared with i.v. administration of DADLE alone. In terms of elimination pathways, metabolism by endopeptidases was the major route for DADLE elimination, whereas rapid biliary excretion was the major route of elimination for the prodrugs. The rapid elimination of the prodrugs by the liver and the formation of stable intermediates after esterase hydrolysis limited the bioconversion efficiencies of the prodrugs to DADLE after i.v. administration. The substrate activity of the prodrugs for efflux transporters (e.g., P-glycoprotein) in the blood-brain barrier significantly restricted their access to the brain.

Acyloxyalkoxy-based cyclic prodrugs of opioid peptides: evaluation of the chemical and enzymatic stability as well as their transport properties across Caco-2 cell monolayers

PURPOSE

To evaluate the chemical and enzymatic stability, as well as the cellular permeation characteristics, of the acyloxyalkoxy-based cyclic prodrugs 1 and 2 of the opioid peptides [Leu5]-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) and DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), respectively.

METHODS

The rates of conversion of 1 and 2 to [Leu5]-enkephalin and DADLE, respectively, were measured by HPLC in HBSS, pH = 7.4, and in various biological media (e.g., human plasma and Caco-2 cell and rat liver homogenates) having measurable esterase activity. The cellular permeation and metabolism characteristics of [Leu5]-enkephalin, DADLE and the cyclic prodrugs 1 and 2 were measured using Caco-2 cell monolayers grown onto microporous membranes and monitored by HPLC.

RESULTS

Cyclic prodrugs 1 and 2 degraded slowly but stoichiometrically to [Leu5]-enkephalin and DADLE, respectively, in HBSS, pH = 7.4. In homogenates of Caco-2 cells and rat liver, as well as 90% human plasma, the rates of disappearance of the cyclic prodrugs were significantly faster than in HBSS. The stabilities of the cyclic prodrugs 1 and 2 were increased significantly in 90% human plasma and Caco-2 cell homogenates when paraoxon, a potent inhibitor of serine-dependent esterases, was included in the incubation mixtures. A similar stabilizing effect of paraoxon was not observed in 50% rat liver homogenates, but was observed in 10% homogenates of rat liver. When applied to the AP side of a Caco-2 cell monolayer, DADLE and cyclic prodrugs 1 and 2 exhibited significantly greater stability than [Leu5]-enkephalin. Based on their physicochemical properties (i.e., lipophilicity), cyclic prodrugs 1 and 2 should have exhibited high permeation across Caco-2 cell monolayers. Surprisingly, the AP-to-BL apparent permeability coefficients (P(App)) for cyclic prodrugs 1 and 2 across Caco-2 cell monolayers were significantly lower than the P(App) value determined for the metabolically stable opioid peptide DADLE. When the P(App) values for cyclic prodrugs 1 and 2 crossing Caco-2 cell monolayers in the BL-to-AP direction were determined, they were shown to be 36 and 52 times greater, respectively, than the AP-to-BL values.

CONCLUSIONS

Cyclic prodrugs 1 and 2, prepared with an acyloxyalkoxy promoiety, were shown to degrade in biological media (e.g., 90% human plasma) via an esterase-catalyzed pathway. The degradation of cyclic prodrug 1, which contained an ester formed with an L-amino acid, degraded more rapidly in esterase-containing media than did prodrug 2, which contained an ester formed with a D-amino acid. Cyclic prodrugs 1 and 2 showed very low AP-to-BL Caco-2 cell permeability, which did not correlate with their lipophilicities. These low AP-to-BL permeabilities result because of their substrate activity for apically polarized efflux systems.

Coumarinic acid-based cyclic prodrugs of opioid peptides that exhibit metabolic stability to peptidases and excellent cellular permeability

To evaluate the cellular permeation characteristics and the chemical and enzymatic stability of coumarinic acid-based cyclic prodrugs 1 and 2 of the opioid peptides [Leu5]-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) and DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), respectively.

METHODS

The rates of conversion of the cyclic prodrugs 1 and 2 to [Leu5]-enkephalin and DADLE, respectively, in HBSS, pH 7.4 (Caco-2 cell transport buffer) and in various biological media having measurable esterase activity were determined by HPLC. The cell permeation characteristics of [Leu5]-enkephalin, DADLE and cyclic prodrugs 1 and 2 were measured using Caco-2 cell monolayers grown onto microporus membranes and monitored by HPLC.

RESULTS

In HBSS, pH 7.4, cyclic prodrugs 1 and 2 degraded chemically to intermediates that further degraded to [Leu5]-enkephalin and DADLE, respectively, in stoichiometric amounts. In 90% human plasma and rat liver homogenate, the disappearance of cyclic prodrugs 1 and 2 was significantly faster than in HBSS, pH 7.4. The half-lives in 90% human plasma and in rat liver homogenate were substantially longer after pretreatment with paraoxon, a known inhibitor of serine-dependent esterases. When applied to the AP side of a Caco-2 cell monolayer, cyclic prodrug 1 exhibited significantly greater stability against peptidase metabolism than did [Leu5]-enkephalin. Cyclic prodrug 2 and DADLE exhibited similar stability when applied to the AP side of the Caco-2 cell monolayer. Prodrug 1 was 665-fold more able to permeate the Caco-2 cell monolayers than was [Leu5]-enkephalin, in part because of its increased enzymatic stability. Prodrug 2 was shown to be approximately 31 fold more able to permeate a Caco-2 cell monolayer than was DADLE.

CONCLUSIONS

Cyclic prodrugs 1 and 2, prepared with the coumarinic acid promoiety, were substantially more able to permeate Caco-2 cell monolayers than were the corresponding opioid peptides. Prodrug 1 exhibited increased stability to peptidase metabolism compared to [Leu5]-enkephalin. In various biological media, the opioid peptides were released from the prodrugs by an esterase-catalyzed reaction, which is sensitive to paraoxon inhibition.

Species differences for stereoselective hydrolysis of propranolol prodrugs in plasma and liver

Species differences and substrate specificities for the stereoselective hydrolysis of fifteen O-acyl propranolol (PL) prodrugs were investigated in pH 7.4 Tris-HCl buffer and rat and dog plasma and liver subfractions. The (R)-isomers were preferentially converted to propranolol (PL) in both rat and dog plasma with the exception of isovaleryl-PL in rat plasma, although the hydrolytic activities of prodrugs in rat plasma were 5-119-fold greater than those in dog plasma. The prodrugs with promoieties (C(=O)CH(R)CH3) based on propionic acid showed marked preference for hydrolysis of the (R)-enantiomers in plasma from both species (R/S ratio 2.5-18.2). On the other hand, the hepatic hydrolytic activities of prodrugs were greater in dog than rat, especially in cytosolic fractions. The hydrolytic activity was predominantly located in microsomes of the liver in rat, while the cytosol also contributed to hepatic hydrolysis in dog. Hepatic microsomal hydrolysis in dog showed a preference for the (R)-isomers except acetyl- and propionyl-PL. Interestingly, in rat liver all types of prodrugs with substituents of small carbon number showed (S)-preference for hydrolysis. The hydrolyses of (R)- and (S)-isomers of straight chain acyl esters in rat liver microsomes were linearly and parabolically related with the carbon number of substituents, respectively, while these relationships were linear for both isomers in dogs.

Moment analysis of intestinal first-pass metabolism by portal-systemic concentration difference in single conscious rat using 5'-deoxy-5-fluorouridine and 5-fluorouracil as model drug system

Intestinal first-pass metabolism was evaluated in a single conscious rat based on a difference in concentrations of parent drug and its metabolite between the portal and systemic bloods (P-S difference method). 5'-Deoxy-5-fluorouridine (5'-DFUR) and 5-fluorouracil (5-FU) were selected as model drug (prodrug of 5-FU) and metabolite pair. The portal vein and the femoral artery of the rat were cannulated so blood samples could be obtained simultaneously from the two sites. 5'-DFUR (100 mg/kg) was administered intraarterially or orally. Concentrations of 5'-DFUR and 5-FU in the portal and arterial samples were assayed by HPLC. The concentration-time profiles of 5'-DFUR and 5-FU were analyzed by local moment analysis. The extent of systemic bioavailability (F) of 5'-DFUR was estimated to be 75.8%. After oral administration, the local absorption ratio (Fa) and the mean local absorption time (ta) of 5'-DFUR were estimated to be 65.8 +/- 7.3% of dose and 74.0 +/- 21.7 min, respectively. The Fa value was close to F, which suggests that the metabolic conversion from 5'-DFUR to 5-FU is not extensive in the liver. The mean absorption time (MAT), calculated to be 76.3 min, almost coincided with ta, which suggests that the mean hepatic transit time is negligible in this experimental scale. The local absorption ratio of metabolite (Fam) was 6.8 +/- 1.7% of orally administered 5'-DFUR, which means that approximately 7% of 5'-DFUR arrived as 5-FU at the portal system. The mean local absorption time (tam) of 5-FU was estimated to be 75.5 min, which is close to that (74.0 min) of 5'-DFUR. Local moment analysis based on P-S difference enabled simultaneous estimation of the local absorption kinetics of a parent compound and the intestinal generation of metabolites by separating the intestinal first-pass metabolism of a drug from the subsequent disposition through the liver and in the systemic circulation.

Esterase-sensitive cyclic prodrugs of peptides: evaluation of an acyloxyalkoxy promoiety in a model hexapeptide

PURPOSE

To evaluate a cyclic acyloxyalkoxycarbamate prodrug of a model hexapeptide (H-Trp-Ala-Gly-Gly-Asp-Ala-OH) as a novel approach to enhance the membrane permeation of the peptide and stabilize it to metabolism.

METHODS

Conversion to the linear hexapeptide was studied at 37 degrees C in aqueous buffered solutions and in various biological milieus having measurable esterase activities. Transport and metabolism characteristics were assessed using the Caco-2 cell culture model.

RESULTS

In buffered solutions the cyclic prodrug degraded chemically to the linear hexapeptide in stoichiometric amounts. Maximum stability was observed between pH 3-4. In 90% human plasma (t1/2 = 100 +/- 4 min) and in homogenates of the rat intestinal mucosa (t1/2 = 136 +/- 4 min) and rat liver (t1/2 = 65 +/- 3 min), the cyclic prodrug disappeared faster than in buffered solution, pH 7.4 (t1/2 = 206 +/- 11 min). Pretreatment of these media with paraoxon significantly decreased the degradation rate of the prodrug. When applied to the apical side of Caco-2 cell monolayers, the cyclic prodrug (t1/2 = 282 +/- 25 min) was significantly more stable than the hexapeptide (t1/2 = 14 min) and at least 76-fold more able to permeate (Papp = 1.30 +/- 0.15 x 10(-7) cm/s) than the parent peptide (Papp < or = 0.17 x 10(-8) cm/s).

CONCLUSIONS

Preparation of a cyclic peptide using an acyloxyalkoxy promoiety reduced the lability of the peptide to peptidase metabolism and substantially increased its permeation through biological membranes. In various biological media the parent peptide was released from the prodrug by an apparent esterase-catalyzed reaction, sensitive to paraoxon inhibition.