In 1957, Isaacs and Lindenman discovered a substance produced by virally infected cells that interfered with further viral growth. They named it interferon (IFN), and it caused much excitement in the medical scientific community because of its therapeutic promise. The immunity established by interferons came to be known as immediate, non-specific, or innate immunity, in contrast to the closely related, slower onset but longer-term and more specific adaptive immunity provided by antibodies and immune cells. Various forms of interferon were identified in many species, and it was subsequently shown that the interferons also had activity against certain tumors. It was then found that a number of compounds could induce cells to make their own interferon. Among the most potent of these were the double-stranded ribonucleic acids (dsRNA) , and in particular, the synthetic dsRNA poly-IC, which consists of a pair of strands of poly-inosinic and poly-cytidylic acids. DsRNAs are not normally found in mammalian cells, but they are the basic genetic material or are replication byproducts of many viruses. This may help explain their activity in stimulating some of the body's basic host defenses.
Plain Poly-IC itself proved to be ineffective in primates partly because it is rapidly inactivated by natural enzymes. Dr. Hilton Levy then discovered that in combination with poly-lysine, the resulting compound, poly-ICLC, is a very stable dsRNA that is a potent interferon inducer and immunomodulator in man. It is now called Hiltonol® in his memory. Early, short term, high dose cancer trials showed that high dose poly-ICLC could induce very large amounts of interferon and other cytokines in man, but with only modest therapeutic effects and moderate transient toxicity.
However, it became apparent that low dose Hiltonol is a more potent clinical activator of a variety of host defense mechanisms that go well beyond simple induction of interferons and which includes reversal or preemption of certain viral or tumor induced inhibitions, much broader immune stimulation, gene regulatory and specific antiviral, and anticancer effects, with little or no toxicity. The pattern of activated genes closely matches those induced by a live viral infection, and Hiltonol is thus recognized as a ‘reliable and authentic” viral mimic in many species, including humans. Certain of these critical effects are inhibited at the higher doses of Hiltonol used in early clinical cancer trials and it is now believed that these effects may be more important clinically than previously thought. This may help explain the inconsistent results of the early clinical trials with high dose interferons or Hiltonol. The activity of Hiltonol and interferons against both certain viruses and certain cancers also serves to remind us how the body's basic defenses can cut across traditional disease classifications.
Some Mechanisms of Action of Hiltonol®
There are several closely interrelated clinical actions of Hiltonol, any of which (alone or in combination) might be responsible for its activation of host defenses, antitumor and viral protective activity. Among these are 1) its induction of interferons; 2) its broader immune enhancing effect; 3) its catalytic activation of specific dsRNA dependent systems such as oligoadenylate synthetase (OAS) the, the p68 protein kinase (PKR) and the MDA5, and 4) its broad gene regulatory actions.
Interferon induction. Hiltonol was initially developed as an interferon inducer and while this is one of its important mechanisms of action, interferons alone have been disappointing as clinical treatments for various cancers. In addition, the levels of serum interferon induced by low dose IM Hiltonol are themselves relatively modest and have not in the past been associated with antiviral or antitumor action. That said, Interferons play a critical role in the adaptive immune actions of Hiltonol.
Immune modulation. In addition to Interferons, Hiltonol induces a ‘natural mix’ of other cytokines and chemokines, as well activation of natural killer cells, dendritic cells, and of killer T cells (CD8 CTL), and facilitation of CTL infiltration into tumors through its demonstrated effect on vascular endothelial cells. Its recently demonstrated effect on dendritic cells through Toll-like receptors (TLR3) may be especially important as is its enhancement of cross presentation and CTL generation through the cytoplasmic MDA5. Dendritic cells play a critical role in the immune response by recognizing pathogens and presenting their antigens to the immune system. One testimony to their importance is their inhibition by a wide variety of viruses such as poxviruses, influenza, and Ebola viruses, as well as by many cancers. There is now increasing evidence that Hiltonol can reverse this inhibition by a variety of these pathogens.
Not surprisingly, as a viral mimic and ‘danger signal’ Hiltonol also has a potent vaccine-boosting or adjuvant effect, with increased antibody and cellular immune response to antigen. For example, administration of low doses of Hiltonol along with swine flu vaccination in monkeys dramatically accelerates and increases antibody production and decreases the amount of antigen needed in the vaccine. The complex interactions of the dsRNAs and the interferons in this regard are still incompletely understood, yet this seemingly paradoxical dual role of Hiltonol as an antiviral agent and immune enhancer is consistent with its function in establishing an immediate defense system against viral attack while at the same time stimulating the establishment of long term immunity. It is currently in wide use as a therapeutic cancer vaccine adjuvant.
A third action of Hiltonol is a more direct antiviral and antitumor effect mediated by natural killer cells and at least two interferon-inducible nuclear enzyme systems, the 2'5' oligoadenylate synthetase (OAS) and the P68 protein kinase (PKR) . DsRNAs such as Poly-IC catalyze the interferon-induced antiviral state in cells by functioning as obligatory cofactors for OAS, which activates ribonuclease-L, as well as for the PKR, which inhibits initiation of protein synthesis. This may help explain the demonstrated preferential decrease of tumor protein synthesis in vivo by Hiltonol.
The OAS and PKR are very sensitive to dsRNA dose and structure. For example, simple, long-chain dsRNA (as in Hiltonol) is the most potent stimulator of OAS and PKR, while shorter or irregular dsRNA can be inhibitory. Similarly, the PKR is inhibited by too high a dose of dsRNA. Clinically, the OAS response is also maximal at relatively low doses of Hiltonol, and is much diminished at higher doses. Mediation of antitumor action by OAS and/or PKR activation could help further explain why the high doses of Hiltonol used in early cancer trials were relatively ineffective.
Many viruses, including but not limited to adenovirus, pox viruses (vaccinia), ebola virus, foot and mouth virus, influenza, hepatitis, poliovirus, herpes simplex, SV-40, reovirus, and the human immunodeficiency virus (HIV) circumvent host defenses partly by down regulating OAS and/or PKR, and this effect can be reversed in vitro by exogenous dsRNA. A block of either PKR and/or OAS-mediated interferon action might also explain the variable response to interferons seen in both viral infections and cancer. Certain viruses as well as tumors such as malignant gliomas may use this or a similar mechanism to circumvent host defenses and cause disease. Those diseases may thus be among the prime targets for clinical Hiltonol therapy in a regimen that maximizes PKR activation.
Clinical Gene Regulation is a fourth mechanism by which Hiltonol can modify the biologic response and provide therapeutic benefit. Plain poly-IC and poly-ICLC have been shown to up-regulate or down-regulate a broad variety of over 270 genes representing multiple cannonical innate immune systems in both cell culture and in humans. Some of these genes play critical roles in the body's natural defenses against a variety of tumors and infections, and in controlling other cell functions, including protein synthesis, programmed (apoptotic) cell death, cell metabolism, cellular growth, the cytoskeleton and the extracellular matrix. The therapeutic implications of these actions are considerable, but have yet to be fully understood.
Viral protective actions of Hiltonol
A detailed discussion of the antiviral actions of Hiltonol is beyond the scope of this outline. However, there is a considerable literature describing the activity of Hiltonol in a broad variety of viral infections, including poxviruses such as vaccinia, hepatitis, influenza, Ebola virus, herpesvirus, rabies, Japanese encephalitis, West Nile virus, and the human immunodeficiency virus (HIV). For example, recent studies have shown strong protection by a single dose of Hiltonol for as long as eight days in a mouse model of smallpox. Likewise, intranasal Hiltonol can protect mice for as long as 3 weeks from an otherwise lethal dose of either influenza virus or SARS virus. This broad spectrum of activity of Hiltonol thus makes it a promising drug for containment of epidemics of certain new or emerging viruses for which positive identification or vaccine may not be immediately available, such as new strains of influenza, West Nile virus, or possibly SARS.
The therapeutic expectations raised in the medical -scientific community with the discovery of the interferons some 60 years ago have so far been only partially realized. Interferons are now in widespread clinical use for such disparate conditions as certain cancers, certain viral infections, and multiple sclerosis. However, much has also been learned about the mechanisms by which certain other viruses and cancers evade the natural host defenses mounted by the interferon system. It now appears that some of these evasive mechanisms can be circumvented by treatment with dsRNAs such as Hiltonol. Experimental agents such as Hiltonol can thus be expected to show activity in situations in which interferons are inactive or only marginally active. DsRNAs are now also recognized to have multiple biological effects that go well beyond the interferons, including multiple gene regulation, and activation of certain basic immune, antiviral and antitumor host defenses. The full clinical therapeutic implications of these findings, however, will only be elucidated through properly designed clinical trials.