7-Allyl-8-oxoguanosine (loxoribine) inhibits the metastasis of B16 melanoma cells and has adjuvant activity in mice immunized with a B16 tumor vaccine.

We have shown previously that loxoribine exhibits adjuvant activity for B cells, activates natural killer (NK) cells, and enhances the activation of lymphokine-activated killer cells by interleukin-2 (IL-2). In this study, we examined loxoribine for protective effects in a B16 melanoma lung tumor metastasis model. Significant inhibition of B16 metastasis was seen in mice given a single injection of 2 mg loxoribine as late as day 3 of tumor growth but the greatest inhibition (96%) was seen in mice given four injections of loxoribine on alternate days starting the day before tumor injection. In experiments in which both IL-2 and loxoribine were administered, both agents were active when tested alone, but the combination of IL-2 and loxoribine gave significantly greater inhibition of metastasis. Loxoribine partially inhibited the development of tumors in mice that had been depleted of NK cells by the administration of anti-asialo-GM1 or anti-NK1.1 antibodies and in NK-deficient beige mice. In all cases, protection was seen only when smaller tumor inocula were injected. Taken together, these data suggest that both NK and non-NK cell populations or effector mechanisms with antitumor activity were activated by loxoribine. Since substituted guanosine analogs have been shown to have adjuvant activity in B cell systems, we evaluated whether loxoribine was active as an adjuvant in a tumor protection model. Mice immunized with both irradiated tumor cells and loxoribine developed a significantly lower number of lung tumors when challenged by live B16 tumor cells, whereas mice injected with either vaccine or loxoribine alone were not protected. There was a clear dose response seen with both loxoribine and the vaccine preparations. These data suggest that loxoribine may be useful in tumor therapy as an immunomodulator or as an adjuvant for use with tumor vaccines.

In vivo activation of natural killer cells and priming of IL-2 responsive cytolytic cells by loxoribine (7-allyl-8-oxoguanosine).

Guanine ribonucleosides which have been substituted at the N7 and/or C8 positions have been shown previously to activate natural killer (NK) cells and to act as sparing agents for interleukin-2 (IL-2) in the in vitro generation of lymphokine activated killer (LAK) cells. In this paper we examined a disubstituted guanosine, 7-allyl-8-oxoguanosine (loxoribine), for the ability to activate NK cells and to interact with IL-2 in the generation of LAK cells in vivo. Following iv administration, loxoribine enhanced murine splenic NK activity in a dose-related fashion, with optimal responses occurring at 3 mg/mouse. Enhanced lysis of YAC-1 cells was seen within 6 hr of injection and NK activity remained elevated for over 96 hr. Mature B and T cells were not required for NK activation since SCID mice responded to loxoribine within the same dose range as did the normal, immunocompetent mice. Both effector and precursor cells were eliminated by the administration of anti-asialo GM1 antibodies and NK activation was totally blocked in mice injected with anti-NK 1.1 antibodies. To test whether loxoribine would act as a sparing agent for IL-2 stimulated LAK activation, mice were injected with 2 mg loxoribine followed by twice daily administration of 10,000 units IL-2. In assays performed 48, 72, and 96 hr after injection of loxoribine, the cytolytic activity with the combination therapy exceeded the activity expected from the algebraic sum of the responses to the individual agents. Single injections of 2 mg loxoribine and 25,000 units IL-2 also stimulated NK/LAK activity, but the greatest enhancement was seen when loxoribine was administered 24 hr before the IL-2. Analysis of mRNA transcripts for the alpha chain of the IL-2 receptor indicated that gene transcription was enhanced within hours of loxoribine administration.

In vivo enhancement of murine natural killer cell activity by 7-allyl-8-oxoguanosine (loxoribine).

7-Allyl-8-oxoguanosine (loxoribine) is a novel immunostimulatory compound which has been shown previously to enhance the antibody synthesis of antigen-stimulated B-lymphocytes. In this report, loxoribine was tested for the ability to activate murine natural killer (NK) cells. In studies in which mice were given a single subcutaneous (s.c.) or intravenous (i.v.) injection of loxoribine, splenic NK cell activity was increased in a dose-related manner with clear enhancement seen within 2 h of drug administration. The enhancement was optimal at 48 h but persisted for a minimum of 4 days. Slow and continuous administration of loxoribine via subcutaneously implanted infusion pumps successfully enhanced the NK activity for several days after all of the pump contents had been delivered. Peak NK responses were seen following s.c. or i.v. administration of 2-3 mg loxoribine per mouse in sesame oil, intralipid, or saline vehicles. Significant oral activity was seen after the administration of 8-10 mg/mouse in sesame oil or intralipid. The in vivo enhancement of NK activity was observed in spleen, blood, and bone marrow but was negligible in lymph nodes and thymus. Multiple injections of optimal concentrations of loxoribine did not tend to enhance the NK activity above that seen with a single injection, suggesting that the timing of injections was critical for optimal responsiveness.