Paul L. Black

Chemoimmunotherapy with cyclophosphamide and bestatin in experimental metastasis in mice

SummaryBestatin has significant therapeutic activity (even following oral administration) for the treatment of metastatic disease, an activity which is limited by tumor burden. Therefore, the therapeutic potential of bestatin was examined in combination with chemotherapy to determine if there is additive activity for heavy tumor burdens. Bestatin significantly increased therapeutic activity and decreased the myelotoxicity of cyclophosphamide following a single injection of cyclophosphamide or split daily doses. In immune function studies, in tumor-bearing antimals, bestatin increased the number of colony-forming units (granulocyte-macrophage) (CFU) and alveolar macrophage tumoricidal activity. However, when bestatin was combined with cyclophosphamide, which depressed bone marrow and macrophage activity, it did not show apparent augmentation of macrophage and NK cell activity, but did significantly increase bone marrow CFU activity. Thus, in combined chemoimmunotherapy, bestatin appears to enhance therapeutic activity by accelerating the recovery of hematopoiesis. We suggest, therefore, that a combination chemotherapy protocol, with oral bestatin, may facilitate myelorestoration following aggressive chemotherapy. The majority of biological response modifiers require parental administration; thus, the identification of an orally active, synthetic immunoaugmenting agent with a defined receptor is of particular interest.

Hematopoietic and hematologic properties of bestatin in normal and cyclophosphamide myelosuppressed mice

Bestatin is a potent inhibitor of aminopeptidase, an enzyme found in abundance in the membrane of monocytes and macrophages. Following binding of bestatin to cells in the histiocytic lineage, the production of colony stimulating activity is upregulated (both in vitro and in vivo) with subsequent increases in hematopoietic and hematologic parameters. We report that the frequency and absolute numbers of CFU-GM as well as entry of CFU-GM into S phase (a measure of progenitor cell activity) is upregulated by treatment of animals with bestatin. This results in an increase in bone marrow cellularity in cyclophosphamide suppressed mice and an increase in the absolute neutrophil count in normal and suppressed mice. The therapeutic application of this hematopoietic modulator has been demonstrated in combination cyclophosphamide and bestatin therapeutic protocols. Because this indication for bestatin has only recently been recognized, few clinical studies have been undertaken utilizing appropriate surrogates of hematopoietic activity. However, preliminary clinical evidence of hematopoietic activity by this non-toxic dipeptide, as reviewed here, suggests that this may be an appropriate strategy for the treatment of myelosuppressed patients.

Imexon and biological response modifiers in murine models of aids

The Rauscher murine leukemia retrovirus system provides an in vivo model of the human acquired immune deficiency syndrome for testing the ability of antiviral agents and biological response modifiers (BRM) to suppress viremia and retroviral disease. In the present report we examined three agents in the Rauscher retrovirus model: imexon, Ampligen and poly[I,C]-LC. Imexon reduced splenomegaly, viremia, and serum reverse transcriptase levels even when treatment was not initiated until 7 days after virus infection. Imexon also significantly prolonged the survival of infected mice. Thus it proved to be an effective antiviral agent in this system, although imexon did not completely eliminate retroviral infection in treated mice. Poly[I,C]-LC and Ampligen had immunomodulatory effects. Both of these BRM augmented the cytolytic activity of splenic natural killer (NK) cells in infected animals when treatment was initiated 24 h after infection. Poly[I,C]-LC had antiretroviral activity when administered on this schedule. In order to examine the role of NK cell augmentation in the antiviral activity of poly[I,C]-LC, we attempted to deplete NK activity by treatment with rabbit antibody to asialo GM1, a ganglioside on the surface of murine NK cells. Combined treatment of infected mice with poly[I,C]-LC and anti-asialo GM1 decreased the antiviral activity of poly[I,C]-LC. This finding suggests that NK cells may be involved in the antiviral effect of this BRM.

Hematopoietic and therapeutic properties of bestatin in normal and myelosuppressed mice

Bestatin is a potent inhibitor of aminopeptidase B, an enzyme which is found in abundance in the membrane of monocytes and macrophages. Binding of Bestatin to cells in the histiocytic linage upregulates colony stimulating activity (both in vitro and in vivo), which subsequently increases hematopoietic and hematologic values. We report that the treatment of mice with Bestatin upregulates the frequency and absolute numbers of colony forming unit--granulocyte-macrophage (CFU-GM), as well as the entry of CFU-GM into S phase (a measure of progenitor cell activity). As a result, there is an increase in bone marrow cellularity in cyclophosphamide myelosuppressed mice and an increase in the absolute neutrophil count in normal and myelosuppressed mice. The therapeutic application of this hematopoietic modulator has been demonstrated in combination cyclophosphamide and Bestatin protocols. While Bestatin has significant therapeutic activity for minimal metastatic disease, therapy models in which the hosts have greater metastatic tumor burdens requires combination chemoimmunotherapy for a significant therapeutic effect. Because myelosuppression as a therapeutic indication for Bestatin has only recently been recognized, few clinical studies have been undertaken with appropriate surrogates of hematopoietic activity. However, the preliminary clinical evidence of hematopoietic activity by this non-toxic dipeptide, as reviewed here, suggests that this may be an appropriate drug for the treatment of myelosuppressed patients. Thus, Bestatin as an orally active biological response modifier (BRM) has significant therapeutic activity for metastatic disease via multiple mechanisms including hematopoietic stimulation and macrophage activating properties.

Evaluation of Immunomodulatory and Therapeutic Properties of Biological Response Modifiers: A Comparison of Preclinical and Clinical Studies

Biological response modifiers (BRMs) are those agents or approaches that modify the relationship between the tumor and host by modifying the host’s biological response to the tumor cells with resultant therapeutic effects (1). BRMs may modify the host responses in several ways: 1. increase the host’s antitumor responses through augmentation and/or restoration of effector mechanisms or mediators of the host’s reaction which may be deleterious; 2. increase the host’s defenses by the administration of natural biologicals (or the synthetic-recombinant derivatives thereof) as effectors or mediators of an antitumor response; 3. augment the sensitivity of the host’s tumor cells to endogenous mechanisms for the control of tumor growth; 4. alter the transformed phenotype by increasing the differentiation/maturation of tumor cells; 5. increase the ability of the host to tolerate damage by cytotoxic modalities of cancer treatment.

Comparison of Therapeutic Potential of Cytokines

Biological response modifiers (BRMs) are those agents or approaches that influence the relationship between the tumor and host by modifying the hosts response to tumor cells, with resultant therapeutic activity (9). A series of studies with recombinant cytokines designed to better understand their immunomodulatory and therapeutic properties have been undertaken. The present report discusses recombinant murine interferon-gamma (rM IFN-g), recombinant human tumor necrosis factor (rH TNF), rH interleukin-2 (rH IL-2), and rM colony stimulating factor-gm (rM CSF-gm). We discuss the development of Optimal Therapeutic Protocols (OTP) for preclinical studies and initial clinical trials that have been initiated to test the resultant clinical hypotheses.