Metabolism of L-[guanidinooxy-14C]canavanine in the rat

The Mechanism of l-Canavanine Cytotoxicity: Arginyl tRNA Synthetase as a Novel Target for Anticancer Drug Discovery

There is a clear need for agents with novel mechanisms of action to provide new therapeutic approaches for the treatment of pancreatic cancer. Owing to its structural similarity to L-arginine, L-canavanine, the beta-oxa-analog of L-arginine, is a substrate for arginyl tRNA synthetase and is incorporated into nascent proteins in place of L-arginine. Although L-arginine and L-canavanine are structurally similar, the oxyguanidino group of L-canavanine is significantly less basic than the guanidino group of L-arginine. Consequently, L-canavanyl proteins lack the capacity to form crucial ionic interactions, resulting in altered protein structure and function, which leads to cellular death. Since L-canavanine is selectively sequestered by the pancreas, it may be especially useful as an adjuvant therapy in the treatment of pancreatic cancer. This novel mechanism of cytotoxicity forms the basis for the anticancer activity of L-canavanine and thus, arginyl tRNA synthetase may represent a novel target for the development of such therapeutic agents.

Canavanine-induced longevity in mice may require diets with greater than 15.7% protein

BACKGROUND: Dietary administration of 1% canavanine had been shown to improve survival in female BALB/c mice consuming diets containing 23.4% protein (dry matter basis). METHODS: In order to determine if this effect also obtains at more moderate dietary protein concentrations, 30 female BALB/c mice were fed a basal diet with 14% protein (15.7% dry matter basis) and another 30 were fed the same diet plus 1% canavanine. RESULTS: Neither mean (Control 873.2 d, Canavanine 870.0 d; SEM = 34.2 d; P = 0.949 from ANOVA) nor median (Control 902 d, Canavanine 884.5 d; P = 0.9058 from Mann-Whitney) lifespans differed between groups.Although mean antinuclear antibody (ANA) titers did not differ between control and canavanine-treated mice at 833 days of age (19.84 vs 20.39 respectively; SEM = 2.64; P = 0.889 from ANOVA), one canavanine-treated mouse displayed an outlying ANA value of 50 (next lower value = 30) denoting possible early sign of incipient autoimmune disease in that individual. CONCLUSION: There may be an interaction between dietary protein level and canavanine with respect to lifespan in mice.

Evaluation of rat intestinal absorption data and correlation with human intestinal absorption

The absorption of 111 drug and drug-like compounds was evaluated from 111 references based on the ratio of urinary excretion of drugs following oral and intravenous administration to intact rats and biliary excretion of bile duct-cannulated rats. Ninety-eight drug compounds for which both human and rat absorption data were available were selected for correlation analysis between the human and rat absorption. The result shows that the extent of absorption in these two species is similar. For 94% of the drugs the absorption difference between humans and rats is less than 20% and for 98% of drugs the difference is less than 30%. There is only one drug for which human absorption is significantly different from rat absorption. The standard deviation is 11% between human and rat absorption. The linear relationship between human and rat absorption forced through the origin, as determined by least squares regression, is %Absorption (human)=0.997%Absorption (rat) (n=98, SD=11). It is suggested that the absorption in rats could be used as an alternative method to human absorption in pre-clinical oral absorption studies.

The natural abundance of L-canavanine, an active anticancer agent, in alfalfa, medicago sativa (L.)

L-Canavanine, a potentially toxic antimetabolite of L-arginine that is stored by many leguminous plants, has demonstrative antineoplastic activity against a number of animal-bearing carcinomas and cancer cell lines. This investigation evaluated the natural abundance of this anti-cancer compound in commercially available sprouts, and in ten varieties of the seed of alfalfa, Medicago Sativa (L.). Canavanine abundance in commercially grown sprouts varied according to the source; the young plant stored appreciable canavanine that ranged from 1.3 to 2.4% of the dry matter. Alfalfa seeds were also rich in this nonprotein amino acid as the canavanine content varied from 1.4 to 1.8% of the dry matter. On average, the tested seeds contained 1.54 ± 0.03% canavanine. Alfalfa seed canavanine content was comparable to the levels found in the seeds of representative members of the genus Canavalia , which are amongst the more abundance sources of this antimetabolite.

L-canavanine, an inhibitor of inducible nitric oxide synthase, improves venous return in endotoxemic rats

OBJECTIVE

To investigate the hemodynamic effects of L-canavanine (an inhibitor of inducible, but not of constitutive, nitric oxide synthase) in endotoxic shock.

DESIGN

Controlled, randomized, experimental study.

SETTING

Animal laboratory.

SUBJECTS

Wistar rats.

INTERVENTIONS

Rats were anesthetized with pentobarbital, and hemodynamically monitored. One hour after an intravenous challenge with 5 mg/kg of Escherichia coli endotoxin, the rats were randomized to receive a continuous infusion of either L-canavanine (20 mg/kg/hr; n = 8) or vehicle only (isotonic saline, n = 11). In all animals, the infusion was given over 5 hrs at a rate of 2 mL/kg/hr. These experiments were repeated in additional rats challenged with isotonic saline instead of endotoxin (sham experiments).

MEASUREMENTS AND MAIN RESULTS

Arterial blood pressure, heart rate, thermodilution cardiac output, central venous pressure, mean systemic filling pressure, urine output, arterial blood gases, blood lactate concentration, and hematocrit were measured. In sham experiments, hemodynamic stability was maintained throughout and L-canavanine had no detectable effect. Animals challenged with endotoxin and not treated with L-canavanine developed progressive hypotension and low cardiac output. After 6 hrs of endotoxemia, both central venous pressure and mean systemic filling pressure were significantly below their baseline values, indicating relative hypovolemia as the main determinant of reduced cardiac output. In endotoxemic animals treated with L-canavanine, hypotension was less marked, while cardiac output, central venous pressure, and mean systemic filling pressure were maintained throughout the experiment. L-canavanine had no effect on the time-course of hematocrit. L-canavanine significantly increased urine output and reduced the severity of lactic acidosis.

CONCLUSIONS

Six hours after an endotoxin challenge in rats, low cardiac output develops, which appears to be primarily related to relative hypovolemia. L-canavanine, a selective inhibitor of the inducible nitric oxide synthase, increases the mean systemic filling pressure, thereby improving venous return, under these conditions.

L-canaline: a potent antimetabolite and anti-cancer agent from leguminous plants

L-Canaline, the L-2-amino-4-(aminooxy)butyric acid structural analog of L-ornithine' is a powerful antimetabolite stored in many leguminous plants. This nonprotein amino acid reacts vigorously with the pyridoxal phosphate moiety of vitamin B6-containing enzymes to form a covalently-bound oxime that inactivates, often irreversibly, the enzyme. Canaline is not only capable of inhibiting ornithine-dependent enzymic activity, but it also can function as a lysine antagonist. Recently, this natural product was found to possess significant antineoplastic in vitro activity against human pancreatic cancer cells.

Toxicity and pharmacokinetics of the nonprotein amino acid L-canavanine in the rat

The toxicity of L-canavanine was investigated because of its demonstrated potential as an antitumor drug. This natural product was only slightly toxic to Sprague-Dawley rats following a single sc injection: the LD50 was 5.9 +/- 1 8 g/kg in adult rats and 5.0 +/- 1.0 g/kg in 10-day-old rats. Following a single dose of 2.0 g/kg, the systemic clearance value for canavanine in adult rats was 0.114 liter/hr, the volume of distribution at steady state was 0.154 liter, and the half-life was 1.56 hr. Forty-eight percent of the dose was excreted unaltered in the urine following an iv injection, and 16% of a sc dose was recovered in the urine. Bioavailability of a 2.0 g/kg sc dose was 72%. Single oral doses of canavanine were less toxic to adult rats than sc injections. Bioavailability of a 2.0 g/kg po dose was 43%, and only 1% of the administered canavanine was recovered in the urine. Twenty-one percent of the administered canavanine remained in the gastrointestinal tract 24 hr after an oral dose. Less than 1% of a 2.0 g/kg dose of L-[guanidinooxy-14C]canavanine was incorporated into the proteins of adult and neonatal rats 4 or 24 hr following administration. Repeated sc administration of canavanine resulted in more severe toxicity. Weight loss and alopecia were observed in rats given daily sc canavanine injections for 7 days. Food intake was decreased by 80% in adult rats subjected to this dosing regimen, but returned to normal after canavanine injections were terminated. Histological studies of tissues from adult rats treated with 3.0 g/kg canavanine daily for 6 days revealed pancreatic acinar cell atrophy and fibrosis. Serum amylase and lipase levels were elevated following one sc injection of 2.0 g/kg canavanine; after three daily injections both serum enzymes were depleted. Elevations in serum glucose and urea nitrogen, and depletion of cholesterol, were observed. The most significant changes were severe attenuations of serum aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase activity.