Physicochemical determinants in hepatic extraction of small peptides

Enhanced tumor retention of NTSR1-targeted agents by employing a hydrophilic cysteine cathepsin inhibitor

We explored the approach of using an analog of E-64, a well-known and hydrophilic cysteine cathepsin (CC) inhibitor, as a potent cysteine cathepsin-trapping agent (CCTA) to improve the tumor retention of low-molecular-weight, receptor-targeted radiopharmaceuticals. The synthesized hydrophilic CCTA-incorporated, NTSR1-targeted agents demonstrated a substantial increase in cellular retention upon uptake into the NTRS1-positive HT-29 human colon cancer cell line. Similarly, biodistribution studies using HT-29 xenograft mice revealed a significant and substantial increase in tumor retention for the CCTA-incorporated, NTSR1-targeted agent. The intracellular trapping mechanism of the CCTA-incorporated agents by macromolecular adduct formation was confirmed using multiple in vitro and in vivo techniques. Furthermore, utilization of the more hydrophilic CCTA greatly increased the hydrophilicity of the resulting NTSR1-targeted constructs leading to substantial decreases in most non-target tissues in contrast to our previously reported dipeptidyl acyloxymethyl ketone (AOMK) constructs. This work further confirms that the CCTA trapping approach can make significant improvements in the clinical potential of NTSR1-and other receptor-targeted radiopharmaceuticals.

Lipophilicity prediction of peptides and peptide derivatives by consensus machine learning

Lipophilicity prediction is routinely applied to small molecules and presents a working alternative to experimental log P or log D determination. For compounds outside the domain of classical medicinal chemistry these predictions lack accuracy, advocating the development of bespoke in silico approaches. Peptides and their derivatives and mimetics fill the structural gap between small synthetic drugs and genetically engineered macromolecules. Here, we present a data-driven machine learning method for peptide log D 7.4 prediction. A model for estimating the lipophilicity of short linear peptides consisting of natural amino acids was developed. In a prospective test, we obtained accurate predictions for a set of newly synthesized linear tri- to hexapeptides. Further model development focused on more complex peptide mimetics from the AstraZeneca compound collection. The results obtained demonstrate the applicability of the new prediction model to peptides and peptide derivatives in a log D 7.4 range of approximately -3 to 5, with superior accuracy to established lipophilicity models for small molecules.

CCK-58 elicits both satiety and satiation in rats while CCK-8 elicits only satiation

Reduction of food intake by exogenous cholecystokinin (CCK) has been demonstrated primarily for its short molecular form, CCK-8. Mounting evidence, however, implicates CCK-58 as a major physiologically active CCK form, with different neural and exocrine response profiles than CCK-8. In three studies, we compared meal-pattern effects of intraperitoneal injections CCK-8 vs. CCK-58 in undeprived male Sprague-Dawley rats consuming sweetened condensed milk. In study 1, rats (N=10) received CCK-8, CCK-58 (0.45, 0.9, 1.8 and 3.6 nmol/kg) or vehicle before a 4-h test-food presentation. At most doses, both CCK-8 and CCK-58 similarly reduced meal size relative to vehicle. Meal-size reduction prompted a compensatory shortening of the intermeal interval (IMI) after CCK-8, but not after CCK-58, which uniquely increased the satiety ratio (IMI/size of the preceding meal). In the second study, lick patterns were monitored after administration of 0.9 nmol/kg CCK-58, CCK-8 or vehicle. Lick cluster size, lick efficiency and interlick-interval distribution remained unaltered compared to vehicle, implying natural satiation, rather than illness, following both CCK forms. In study 3, threshold satiating doses of the two CCK forms were given at 5 and 30 min after meal termination, respectively. CCK 58, but not CCK-8 increased the intermeal interval and satiety ratio compared to vehicle. In conclusion, while CCK 58 and CCK-8 both stimulate satiation, thereby reducing meal size, CCK-58 consistently exerts a satiety effect, prolonging IMI. Given the physiological prominence of CCK-58, these results suggest that CCK's role in food intake regulation may require re-examination.

Acyclovir prodrug for the intestinal di/tri-peptide transporter PEPT1: comparison of in vivo bioavailability in rats and transport in Caco-2 cells

It has previously been shown that the prodrug Glu(acyclovir)-Sar has a high affinity for PEPT1 in Caco-2 cells. However, affinity does not necessarily lead to translocation by the transporter which is necessary for achieving an increased oral bioavailability. Therefore i.v. and p.o. doses of Glu(acyclovir)-Sar, acyclovir and valacyclovir were given to rats and the collected blood samples were analysed via LC-MS-MS. Furthermore, Caco-2 cell monolayers were exposed apically to Glu(acyclovir)-Sar, acyclovir, and valacyclovir and the concentration of drug and prodrugs in the cell extracts were determined and taken as a measure for intracellular accumulation. In addition, bi-directional transport studies of Glu(acyclovir)-Sar across Caco-2 cell monolayers and in vitro metabolism studies of Glu(acyclovir)-Sar in various media of rat origin were performed. For these purposes HPLC-UV analysis was applied. Oral administration of Glu(acyclovir)-Sar to rats resulted in low bioavailabilities of acyclovir (<2%) and intact prodrug (<5%). Studies performed on Caco-2 cell monolayers showed that in contrast to valacyclovir Glu(acyclovir)-Sar did not result in a detectable amount of acyclovir or Glu(acyclovir)-Sar in the cell extracts. Bi-directional flux across Caco-2 cell monolayers apical to basolateral (FluxA-->B) and basolateral to apical (FluxB-->A) was measured and the FluxB-->A/FluxA-->B ratios of approximately 0.8 indicate that apical efflux mechanisms may not explain this lack of intracellular accumulation. These data indicate that Glu(acyclovir)-Sar may not be translocated by PEPT1.

Influence of structural variations in peptidomimetic 4-amidinophenylalanine-derived thrombin inhibitors on plasma clearance and biliary excretion in rats

PURPOSE

Systemic and hepato-biliary clearance of peptidomimetic thrombin inhibitors of the 4-amidinophenylalanine amide-type, derived from NAPAP (Nalpha-[2-naphthylsulfonyl-glycyl]-4-amidinophenylalanine-piperidide) by substituting Gly in P2 for natural and unnatural amino acids or by varying the C- and N-terminal moieties. resp., were investigated.

METHODS

Concentrations of the compounds administered as intravenous bolus injection at a dose of 1 mg/kg to bile duct-cannulated rats were determined in plasma and bile samples collected over 4 hours using reversed-phase HPLC.

RESULTS

NAPAP and the derivatives with additional charged groups are comparatively hydrophilic compounds. For NAPAP and most of the derivatives the biliary clearance accounted for a high percentage of the rapid systemic plasma clearance. Derivatives 2a-c with a second basic group in P2 position showed lower systemic and biliary clearance compared to NAPAP, whereas their cumulative biliary excretion after a period of 120 min was less affected. Bis-benzamidine derivatives 4a and 5 with the second amidino group in the N-terminal moiety had the lowest biliary clearance. Additional carboxylic groups reduced the systemic and biliary clearance only as free amidinophenylalanine carboxyl in 3a and 5. No influence compared to NAPAP was observed for 2d with a free carboxyl group in P2 position.

CONCLUSIONS

The weak correlation of the log P values of the compounds with the clearance parameters indicates the influence of structural variations, especially of charged groups, in this series of compounds rather than overall lipophilicity on hepato-biliary elimination mediated by hepatocellular transporters.

Effect of exogenous cholecystokinin (CCK)-8 on food intake and plasma CCK, leptin, and insulin concentrations in older and young adults: evidence for increased CCK activity as a cause of the anorexia of aging

Healthy aging is associated with reductions in appetite and food intake--the so-called anorexia of aging, which may predispose to protein-energy malnutrition. One possible cause of the anorexia of aging is an increased satiating effect of cholecystokinin (CCK). To investigate the impact of aging on the satiating effects of CCK, 12 young and 12 older healthy subjects received 25-min iv infusions of saline (control) and CCK-8, 1 ng/kg per min or 3 ng/k per min, on 3 separate days before a test meal. Older subjects ate less than young subjects, and food intake was suppressed 21.6% by CCK-8, compared with the control day (P < 0.05). The suppression of energy intake by CCK-8 in older subjects was twice that in young subjects (32 +/- 6% vs. 16 +/- 6% SEM, P < 0.05) and was related to plasma CCK-8 concentrations, which were higher at baseline (P < 0.05) and increased more during CCK-8 infusions in older than young subjects (P < 0.01). The extent of suppression of food intake per given rise in plasma CCK-8 concentrations did not differ between the two age groups (P = 0.35). Endogenous CCK concentrations were higher at baseline in older subjects (P < 0.001) and decreased during the CCK-8 but not control infusions (P < 0.01), suggesting that CCK suppresses its own release. Plasma leptin concentrations were not affected by CCK infusion, whereas postprandial insulin concentrations were lowered and the peak postprandial glucose concentration was delayed but not affected by CCK-8 infusion. Because older people retain their sensitivity to the satiating effects of exogenous CCK and plasma endogenous CCK concentrations are higher in older people, increased CCK activity may contribute to the anorexia of aging.

Octanol-water partition of nonzwitterionic peptides: predictive power of a molecular size-based model

A remarkably simple, molecular size-based model developed to predict octanol-water partition coefficients for organic compounds is tested on a set of 188 neutral peptides with available experimental partition data. Despite using only two parameters, it gives a promising correlation (r2 = 0.914; sigma = 0.455, F = 1978.0), and predictions are in a realistic range even for larger peptides (cyclosporin, melanotan, sandostatin) where common, overparametrized fragment methods become quite unreliable. Ion-pair partitioning and the extraction constant formalism is briefly reviewed to describe the sigmoidal lipophilicity profile of ionizable, nonzwitterionic peptides. It seems possible to extend the present model to estimate apparent partition coefficients measured around neutral pH and physiological conditions for monoionic peptides; however, as no standard conditions are yet defined and only relatively small number of experimental data are available, the situation here is more complex.

Impairment of hepatic transport processes in perfused rat liver by the specific CCK receptor antagonist loxiglumide

The specific cholecystokinin (CCK) receptor antagonist loxiglumide has been used in several human and animal studies to investigate the role of CCK in gastrointestinal physiology. In the present study, the interference of this CCK receptor antagonist with hepatic transport processes was characterized in the perfused rat liver. Indocyanine green, an organic dye which is secreted into bile without being metabolized, was taken up in control experiments at a rate of 68.1 +/- 7.7%. The CCK receptor antagonist lowered the extraction to 0.5 +/- 2.6% (P < 0.001). The compound diminished the hepatic extraction of CCK-8 from 90.95 +/- 2.60% to 4.90 +/- 1.95% (P < 0.001) and of gastrin from 22.2 +/- 1.1% to 8.2 +/- 1.9% (P < 0.001). The hepatic extraction of lidocaine, which is metabolized by the cytochrome P450 system, was only slightly altered. For leukotrienes and taurocholate, the rate-limiting step for transport into bile is secretion across the canalicular membrane; the hepatic extraction of leukotriene D4 was markedly diminished by loxiglumide whereas the transport of taurocholate was only slightly inhibited. The present study demonstrates that the specific CCK receptor antagonist loxiglumide diminished the hepatic extraction of various substances, including peptides and organic anions. It did not interfere with the cytochrome P450 system. The pronounced reduction of hepatic uptake of indocyanine green and leukotriene may be due to an interference with the transport system of these substances in the liver.

Hepatic concentrations of proenkephalin-derived opioids are increased in a rat model of cholestasis

The liver of adult rats with cholestasis secondary to bile duct resection has been shown to express the proenkephalin gene and, by immunohistochemical stains, to contain met-enkephalin. To further study hepatic opioids in cholestasis, concentrations of proenkephalin-derived endogenous opioids were measured in a rat model of cholestasis by the use of radioimmunoassays. The specificity of the immunoreactivity detected by the assays was confirmed by high performance liquid chromatography (HPLC). In adult male rats with cholestasis due to BDR, the concentrations of three proenkephalin-derived opioid peptides were increased. Specifically, the mean hepatic concentrations of met-enkephalin, Met-Enk-Arg6-Phe7 and leu-enkephalin were 2.5 (p < 0.005), 2.1 (p < 0.005) and 2.5 (p < 0.01) fold higher than the corresponding mean for controls. These findings provide further independent evidence that opioid peptides accumulate in the liver in a model of cholestasis and are consistent with de novo synthesis of opioid peptides occurring in the cholestatic liver. This phenomenon may have relevance to the altered function of the opioid system in cholestasis and to the role of the liver as a neuroendocrine organ.

ATP-dependent transport of the linear renin-inhibiting peptide EMD 51921 by canalicular plasma membrane vesicles of rat liver: evidence of drug-stimulatable ATP-hydrolysis

Certain peptide drugs, such as the linear hydrophobic renin-inhibitor EMD 51921, are rapidly eliminated via the bile. At the sinosoidal membrane of liver cells EMD 51921 is taken up via a sodium-independent carrier-mediated mechanism, competing for the uptake of bile acids. Until now, the mechanisms of biliary excretion of EMD 51921 were unknown. In this study we describe an ATP-dependent transport system for the enzymatically and metabolically stable hydrophobic linear renin-inhibiting peptide EMD 51921. The ATP-dependent uptake into the osmotic reactive intravesicular space is saturable (Km 12 microM, Vmax 663 pmol/min per mg protein), temperature dependent and specifically requires ATP. Transport is inhibited by vanadate but not by ouabain, EGTA or NaN3, and does not function in basolateral plasma membrane vesicles. Transport is not altered in canalicular membrane vesicles isolated from Tr- rats lacking the canalicular ATP-dependent transport of cysteinyl leukotrienes and related anions. Transport is inhibited by taurocholate, a typical substrate of the canalicular ATP-dependent bile acid transporter, but also by vincristine and daunomycin, substrates of P-glycoproteins. EMD 51921, however, only inhibits the uptake of taurocholate, whereas the transport of daunomycin is not influenced. Taurocholate and EMD 51921 are mutually non- or un-competitive transport inhibitors. Incubation of rat liver canalicular membranes with micromolar concentrations of EMD 51921 resulted in a 1.8-2.5-fold increase in the rate of ATP-hydrolysis. In contrast, ATP-hydrolysis was not affected by fragments of the peptide that are not transported in an ATP-dependent manner. The apparent Km value (EMD) for ATP-hydrolysis is 68 microM. Vmax is 0.032 U/mg protein. ATPase activity is pH dependent. Stimulation of ATP-hydrolysis is inhibited by vanadate, NEM, hydroxymercuribenzoate and ascorbate, but is not affected by ouabain, EGTA or NaN3. EMD 51921 does not stimulate the ATPase activity of the Na+/K(+)-ATPase isolated from kidney medulla. The EMD-stimulatable ATPase seems to be distinct from the glutathione-S-conjugate stimulatable ATPase and the mdr 1a/b gene products and differs in its characteristics from that of the canalicular ecto-ATPase.

Gastrointestinal absorption and plasma clearance rates of [D-Arg8]vasopressin analogues in the rat

The gastrointestinal absorption of a series of vasopressin (VP) analogues with enhanced enzymatic stability was determined in chronically catheterized, conscious rats. The following peptides were used: [Mpa1,D-Arg8]vasopressin (dDAVP), [Mpa1,Asn4,D-Arg8]VP, [Mpa1,Val4,D-Arg8]VP, [Mpa1,(CH3)3Ala4,D-Arg8]VP, [Mpa1,Tyr(ethyl)2,D-Arg8]VP, and [Mpa1,D-Tyr(ethyl)2,Ile3,Val4,D-Arg8]VP. The peptides were administered by gavage feeding and blood samples were taken repeatedly for 3 h. In another series of experiments, plasma clearance rates (Clp) were determined using the constant infusion method. Plasma concentrations were measured by use of a cross-reacting dDAVP antiserum in a radioimmunoassay method. The bioavailability of all peptides was below 0.1%. The Clp values differed sevenfold; the lowest was for [Mpa1,D-Tyr(ethyl)2,Ile3,Val4,D-Arg8]VP and the highest was for [Mpa1,Asn4,D-Arg8]VP. With the exception of dDAVP the Clp values of the analogues showed an inverse relationship with hydrophilicity. Incubations in relatively concentrated intestinal contents for 1 h showed extensive degradation of the analogues except for [Mpa1,D-Tyr(ethyl)2,Ile3,Val4,D-Arg8]VP. It can be concluded that, in the rat, the bioavailability of dDAVP is lower than in other animal species and in man. Increased resistance to peptide degradation by gastrointestinal contents did not improve absorption. Therefore, the permeability properties of the intestinal mucosa are likely to be a more important factor affecting the gastrointestinal absorption of this group of peptides, although postabsorption events, like hepatic extraction, may also play a role.

Pharmacokinetic properties of the tocolytic agent [Mpa1, D-Tyr(Et)2, Thr4, Orn8]-oxytocin (antocin) in healthy volunteers

OBJECTIVE

The aim of this study was to study the pharmacokinetics of antocin, the tocolytic oxytocin antagonist [Mpa1, D-Tyr2(Et), Thr4, Orn8]-oxytocin.

DESIGN

Antocin was injected intravenously as a bolus dose (5 mumol). Blood samples were taken at intervals for 240 minutes. In addition, the binding of 125I-Tyr10-antocin to blood constituents was determined and compared with 125I-AVP and 125I-[Mpa1, D-Arg8]-vasopressin (desmopressin).

SUBJECTS

Eight healthy, non-smoking adults, three male and five female.

MEASUREMENTS

Antocin was measured using a specific radioimmunoassay after prior extraction of the plasma. Plasma binding was estimated using polyethyleneglycol precipitation.

RESULTS

The rate of plasma disappearance of antocin was best fitted by a biexponential curve. The clearance of antocin was 23.5 +/- 7.6 l/h, the volume of distribution was 13.1 +/- 3.8 l and the biological half-life was 39.0 +/- 4.1 minutes. A greater proportion of 125I-Tyr10-antocin bound to plasma proteins (33.5%) and red blood cells (13%) than did 125I-AVP, 125I-desmopressin and unlabelled desmopressin.

CONCLUSIONS

The half-life was longer and the clearance of antocin was less than that found in a previous study when a non-specific antiserum was used. This is most likely because of the extended blood sampling time period which revealed the biphasic decay pattern. The higher plasma clearance of antocin compared to oxytocin and desmopressin may be explained by its increased binding to blood constituents rather than by differences in enzymatic degradation of the molecules.

Sympathetic nerves, but not the adrenal gland, contribute to elevated plasma levels of met-enkephalin in rats with acute cholestatic hepatitis

Met-enkephalin is known to circulate in human and animal plasma in low levels. However, the source(s) of plasma met-enkephalin have not been completely elucidated. It has been proposed that the adrenal gland, sympathetic nerves, pancreas and the gut might be implicated. Recently, markedly elevated levels of met-enkephalin have been documented in the presence of liver disease. To investigate potential sources of met-enkephalin in liver disease, rats with acute cholestatic hepatitis 24 h after gavage with alpha naphthylisothiocyanate (ANIT) 100 mg/kg were studied. Plasma met-enkephalin levels were determined by radioimmunoassay in plasma samples from normal, adrenalectomized, or chemically sympathectomized animals. In control rats, ANIT treatment resulted in a striking 8.7-fold increase in systemic venous met-enkephalin levels (inferior vena cava) (P < or = 0.0005) and a significant increase in peptidase-derived met-enkephalin levels (determined after trypsin/carboxypeptidase B digestion of plasma samples) (P < or = 0.05). ANIT-treatment also resulted in a 5.6-fold increase in portal vein met-enkephalin levels (P < or = 0.005). Portal vein met-enkephalin levels were only 1.2-fold higher than IVC levels in ANIT-treated rats (P < or = 0.05). Plasma activities of the two main enkephalin degrading enzymes, aminopeptidase and enkephalinase, were similar in control and ANIT-treated rats. Chemical sympathectomy, prior to ANIT treatment, decreased the elevation in inferior vena caval met-enkephalin levels by 35% (P < or = 0.005). Adrenalectomy did not alter ANIT-induced increases in circulating met-enkephalin levels (pNS).(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatocellular uptake of peptides--I. Carrier-mediated uptake of hydrophilic linear peptides with renin inhibitory activity into isolated rat liver cells

The hepatic uptake of a hydrophilic, cationic linear peptide with renin inhibitory activity [5(4-amino-piperidyl-1-carbonyl)-L-2,6[3H]phenyl-alanyl-beta-alanyl-(4S- amino-3S-hydroxy-5-cyclohexyl)-pentan-carbonyl-L-isoleucyl-amin ome thyl-4-amino-2-methyl-pyrimidine-citrat] (code number EMD 56133; EMD, E. Merck, Darmstadt) was investigated in isolated rat hepatocytes. EMD 56133 was taken up by isolated rat liver cells in a time-, concentration-, energy- and temperature-dependent manner. The uptake was a combination of diffusion and a carrier-mediated process. EMD 56133 was accumulated 4.5-fold in liver cells. Eighty-three per cent of the accumulated peptide was found in the cytosol, not bound to membrane proteins. Seventeen per cent was associated with membrane proteins after cell fractionation and centrifugation at 100,000 g. The permeability coefficient of the non-saturable uptake of EMD 56133 was P = 1.973 x 10(-6) cm/sec. The kinetic constants for the carrier-mediated transport are Km = 92 microM and Vmax = 128 pmol/mg x min. Various substrate analogs inhibited the uptake of EMD 56133. AS-30D ascites hepatoma cells and Reuber hepatoma cells did not accumulate EMD 56133. The absence of oxygen or a decreased cellular ATP content blocked the hepatocellular uptake of the renin inhibitor. Temperatures above 20 degrees increased the transport; the activation energy was determined to be Aapp = 41 kJ/mol. The apparently active uptake of EMD 56133 was not sodium dependent. In contrast, the membrane potential might be a driving force for the transport of the positively charged EMD 56133.

Hepatobiliary excretion of bacterial formyl-methionyl peptides in rat. Structure activity studies

The bacterial chemotactic peptide formyl-met-leu-phe and its radioiodinated analog formyl-met-leu-[125I]tyr are rapidly excreted by the liver into bile following portal or systemic venous infusions in rats or after absorption from the gut lumen. To determine the molecular structural requirements for hepatobiliary excretion of formyl-methionyl peptides, structure-activity studies using portal venous infusions of 24 structural analogs of formyl-met-leu-tyr were performed in rats with biliary cannulae. Hepatic extraction of peptides was studied in vivo using external gamma counting after portal infusion. Efficient hepatobiliary excretion was not restricted to bioactive formyl peptides, but showed a broad specificity for different amino-acylated (formyl, acetyl, propionyl, carbobenzoxy) di- and tripeptides and no requirement for methionine in position one or for a free carboxy terminus. However, nonacylated peptides and an acyl-amino acid showed little excretion. Hepatic extraction of peptide was also related to N-acylation. Hepatic extraction and excretion of N-acyl peptides were also related to hydrophobicity. Thus, the presence of an N-acyl group is the key determinant of biliary excretion of inflammatory bacterial f-met peptides in the rat.