Philip Klubes

Uridine rescue from the lethal toxicity of 5-fluorouracil in mice.

SummaryTo determine the relationship between 5-fluorouracil (FUra) toxicity and its RNA- and DNA-directed actions we examined the ability of continuous SC infusion with uridine (Urd), thymidine (dThd), or deoxyuridine (dUrd) to rescue mice from the lethal toxicity of FUra. Male B6D2F1 mice were treated with a single IP injection of FUra (800 mg/kg) followed in 24 h by a 5-day infusion with either 0.9% NaCl or Urd (0.1, 1, 5, or 10 g/kg/day). Survivors were then followed up for 30 days after FUra treatment. Urd (1, 5, or 10 g/kg/day) rescued mice from the lethal toxicity of FUra, whereas Urd (0.1 g/kg/day) was as ineffective as 0.9% NaCl as a rescue agent. With variable doses of FUra followed in 24 h by a Urd infusion (5 g/kg/day) for 5 days, Urd rescued mice treated with FUra (400, 600, or 800 mg/kg) but was ineffective against higher doses of FUra (1,000 or 1,200 mg/kg). Mice treated with FUra (800 mg/kg) followed in 24 h by a 5-day infusion with either dThd (1, 5, or 10 g/kg/day) or dUrd (1 or 5 g/kg/day) could not be rescued from the lethal toxicity of FUra. In all experiments deaths occurred between 6 and 12 days after FUra. These results, which demonstrate a specificity for Urd, but not for either dThd or dUrd, for rescuing mice from the lethal toxicity of FUra, suggest the importance of the RNA- rather than the DNA-directed actions of FUra as a determinant of its toxicity in mice.

Effect of concurrent calcium leucovorin infusion on 5-fluorouracil cytotoxicity against murine L1210 leukemia.

SummaryWe examined the effect of concurrent SC infusion of calcium leucovorin (LV) on the action of 5-fluorouracil (FUra) against mouse L1210 leukemia implanted either SC or IP. Mice bearing the SC tumor treated with FUra (100 mg/kg, IP, day 1) plus infusion with either LV (11.5 mg · kg-1 · day-1, days 1–4), or 0.9% NaCl (days 1–4) resulted in an identical increase in median lifespan (ILS) of 28%. Similar experiments with FUra (100 mg/kg) plus LV infusion (115 mg · kg-1 · day-1) or FUra (200 mg/kg) plus LV infusion (115 mg · kg-1 · day-1) resulted in 50% and 59% ILS, respectively, which were not different from that obtained with the same doses of FUra plus 0.9% NaCl infusion. Mice bearing the IP tumor treated with FUra (100 mg/kg, IP, day 1) plus infusion with either LV (11.5 mg · kg-1 · day-1, days 1–4) or 0.9% NaCl (days 1–4) had an identical 56% ILS. Similar experiments with FUra (100 mg/kg) plus LV infusion (115 mg · kg-1 · day-1) or FUra (200 mg/kg) plus LV infusion (115 mg · kg-1 · day-1) resulted in 67% and 94% ILS, respectively, which were not different from those obtained with the same doses of FUra plus 0.9% NaCl infusion. Treatment of normal mice with FUra (200 mg/kg, IP, day 0) plus LV infusion (115 mg · kg-1 · day-1, days 0–3) was no more toxic than FUra plus 0.9% NaCl infusion, judging by similar transient decreases in body weight and no mortality. The data indicate that concurrent infusion with the LV failed to enhance the action of FUra against the mouse L1210 leukemia.

Attempts to increase intratumoral blood flow in the rat solid walker 256 tumor by the use of the perfluorocarbon emulsion fluosol-DA

We have examined the effect of the perfluorocarbon emulsion, Fluosol-DA 20% (FDA), on blood flow in rats bearing an advanced solid Walker 256 tumor implanted s.c. Blood-FDA exchange in unanesthetized rats maintained under 100% oxygen was accomplished by simultaneous arterial withdrawal and i.v. infusion until the hematocrit was less than 4%. Control rats were maintained under 100% oxygen but did not undergo any exchange. Regional blood flow studies in tumors of control and FDA-exchanged rats were performed using [14C]iodoantipyrine and quantitative autoradiography. FDA-blood exchange did not increase flow to the whole tumor. Similarly, the pattern of regional flow within the tumor, which was determined in histologically distinct areas--including dense and normocellular, necrotic and peripheral zones invading into muscle and connective tissue--was not substantially altered. Flow to cerebral tissue was increased two-fold, although flow to normal tissues including temporalis muscle, skin and diaphragm was not altered. These results show that FDA-blood exchange does not enhance vascular flow in solid Walker 256 tumor implanted s.c. in the rat.

Comparison of the bioavailability of uridine in mice after either oral or parenteral administration

SummaryWe compared the bioavailability of uridine (Urd) (350 and 3500 mg/kg) administered either as a single SC injection or by gavage, in male CD8F1 mice. Plasma samples were analyzed for Urd and uracil (Ura) using high-pressure liquid chromatography. After Urd (3500 mg/kg, SC), plasma Urd levels peaked at 4900 μM and then declined to pretreatment levels (< 10 μM) within 6h. Plasma Ura concentrations peaked at 1400 μM and then declined initially more slowly than Urd. After Urd (3500 mg/kg, PO) plasma levels of Urd were fairly constant (range 33–82 μM) for up to 8 h and had returned to pretreatment levels at 16 h. Plasma Ura concentrations paralleled Urd, but were approxximately ten-fold higher. Areas under the concentration-time curve for Urd showed that the bioavailability of Urd after PO administration was 7% of that after SC administration. After Urd (350 mg/kg, SC) Urd levels peaked at 210 μM returning to pretreatment levels within 2 h. Plasma Ura levels reached a peak with 300 μM and then declined initially more slowly than those of Urd. After Urd (350 mg/kg, PO) plasma Urd levels were not perturbed, although Ura levels peaked at 50 μM after which they declined and could no longer be detected at 4 h. These data indicate that (a) the bioavailability of Urd (350 or 3500 mg/kg) was lower when given PO than when it was administered by SC injection; and (b) Urd (3500 mg/kg) PO resulted in prolonged and relatively constant plasma Urd levels compared with Urd (3500 mg/kg) SC. These results suggest that Urd PO should be compared with parenterally administered Urd in attempts to increase the therapeutic index of 5-fluorouracil and of antimetabolite inhibitors of de novo pyrimidine biosynthesis.

Understanding the actions of carcinostatic drugs to improve chemotherapy: 5-fluorouracil

Abstract The accumulation of additional basic knowledge about 5-fluorouracil is still required in order to improve the optimal utilization of this drug in the cancer patient. The drugs inhibitory effects on ribosome formation, already recognized as an important drug response in microorganisms and rodent tissues, has now been demonstrated for human colon tumors. However, ribosome synthesis does not occur sufficiently rapidly under the conditions of these experiments so that this assay appears to have limited promise as a predictor for human tumor responsiveness to FU. In the L1210 system, a rodent tumor model sensitive to FU, FdUMP was formed rapidly, and this metabolite persisted for several days in the tumor after a single dose of FU. DNA synthesis was inhibited simultaneously in concurrence with peak FdUMP levels, and recovery of this inhibition took place as the tissue concentration of FdUMP declined. DNA synthesis in bone marrow and gastrointestinal tract, which was also inhibited by drug treatment, recovered more rapidly than that of tumor, in line with the antitumor selectivity of FU in this tumor model. For the relatively unresponsive Walker 256 rat tumor, even higher levels of FdUMP were formed initially, but this metabolite was retained only briefly in the tumor after a single dose of FU. Inhibition of DNA formation in tumor was more transient than that in normal tissues, probably accounting for the nonselective antitumor response in that model. It is likely that tissue retention of FdUMP may serve as a major determinant for activity of FU in tumor and normal tissues, but other factors including inhibition of ribosomal maturation also play a role in the drugs cytotoxic action.