Immunoadjuvants for vaccines : key applications

Adjuvants are broadly used in vaccines to enhance and/or direct the specific immune response to antigens. The mechanism of action of adjuvants depends mainly on their physico-chemical properties and/or molecular characteristics [1].

From an immunological aspect, there are two types of vaccine adjuvants [2]: adjuvants exerting their effects by generating an inflammation at the site of injection (such as emulsions and particulate adjuvants, i.e. alum salts), and adjuvants exerting their effects by specifically targeting DCs and monocytes/macrophages at the site of injection (such as ligands of pattern recognition receptors (PRR), i.e. Immunoadjuvants). Some adjuvants can combine both properties [3].

The type of vaccine adjuvant used depends mainly on the type of immune response aimed at [2]. Additionnally, the site of injection, the mode of application and the physico-chemical properties of the formulated vaccine play an essential role [1].

Importantly, vaccine adjuvantation is a science by itself. More information on vaccine adjuvants is available in numerous scientific publications.


RIBOXXOL® CLINIgrade® : immunoadjuvant for 21st century vaccines

Riboxx has developped a new immunoadjuvant called RIBOXXOL®. RIBOXXOL is a ligand of Toll-like-receptor 3. Toll-like-receptor 3 is a PRR located in myeloid dendritic cells, NK cells and monocytes/macrophages [4]. The ligand of TLR3 is double stranded RNA with a length of > 45 bp, independent from the RNA sequence [5]. Ligation of TLR3 by immunoadjuvants such as poly(I:C) has been used for more than 3 decades, and is reported so far in more than 45 clinical trials [6].

The benefits of using TLR3 ligands as immunoadjuvants lies mainly in the fact that classical adjuvants such as alum slats of water-in-oil emulsions induce a strong Th2 immune response, but little or no Th1 immune response. A strong Th1 immune response is important for vaccination against viral infections such as hepatitis, HIV, Dengue and Flu, but also parasites such as Plasmodium Falciparum, or Malaria. Moreover, in therapeutic vaccines such as vaccines against cancer, a Th1 immune response is essential.

Major disadvantages of poly(I:C) comprise its undefined chemical structure and very poor homogeneity. poly(I:C) is composed of a mix of single-stranded and double-stranded RNA molecules ranging from 1.5 to 8 kb, imperfectly annealed as dsRNA or single stranded RNA. This is mainly due to limited solubility and difficult reconstitution of poly(I:C) that requires heating (50-60°C) and slow cooling over many hours to achieve re-annealing of both poly(I) and poly(C) strands. As a consequence, poly(I:C) has a reported toxicity in clinical trials, ranging from hypersensitivity to coagulopathy, renal failure, or systemic cardiovascular failure [7].

A further problem of dsRNA compounds such as poly(I:C) are their rapid degradation in body fluids by RNAses, with a reported half-life of few minutes [8], and subsequent unpredictable pharmacokinetics of degradation products. Optimization of physicochemical properties of poly(I:C) has led to generation of derivatives that have increased stability in body fluids (such as polyICLC), [9] or reduced toxicity through reduced decreased in body fluids (such as poly(I:C12U) [10].

poly(I:C) and its derivatives are produced under GMP conditions for intravenous administration and have been tested in various clinical trials [6].

Riboxx has developped RIBOXXOL, a novel TLR3 ligand  with a very well defined chemical structure, length and molecular weight, a good solubility and serum stability, being able to activate DCs in a dose-dependent manner by specifically targeting endosomal TLR3.


Applications of immunoadjuvants for vaccines

  • Prophylactic vaccines (virus infections)
  • Prophylactic vaccines (bacterial infections)
  • Therapeutic vaccines (tumors, virus infections)



1. Lambrecht BN, Kool M, Willart MA, Hammad H. Mechanism of action of clinically approved adjuvants. Curr Opin Immunol. 2009 Feb;21(1):23-9.
2. De Gregorio E, D'Oro U, Wack A. Immunology of TLR-independent vaccine adjuvants. Curr Opin Immunol. 2009 Jun;21(3):339-45.
3. Baz M, Samant M, Zekki H, et al. Effects of different adjuvants in the context of intramuscular and intranasal routes on humoral and cellular immune responses induced by detergent-split A/H3N2  influenza vaccines in mice. Clin Vaccine Immunol. 2012 Feb;19(2):209-18.
4. Visintin A, Mazzoni A, Spitzer JH et al., "Regulation of toll-like receptors in human monocytes and dendritic cells." J Immunol. 2001 (166):249-55.
5. Botos I, Segal DM, Davies DR. The structural biology of toll-like receptors. Structure 2011 19(4):447-59.
6. Galluzzi L, Vacchelli E, Eggermont A et al. Trial watch: Experimental toll-like receptor agonists for cancer therapy. Oncoimmunology 2012 1(5): 699-716.
7. Robinson RA, DeVita HB, Levy H et al., A phase I-II trial of multiple-dose polyriboinosic-polyribocytidylic acid in patients with leukemia or solid tumors. J Natl Cancer Inst. 1976 57(3):599-602.
8. Bumcrot D, Manoharan M, Koteliansky V et al. RNAi therapeutics: A potential new class of pharmaceutical drugs. Nat Chem Biol. 2006 2(12):711-19.
9. Levy HB, Baer G, Baron S et al. A modified polyriboinosinic-polyribocytidylic acid complex that induces interferon in primates. J Infect Dis. 1975 132(4):434-39.
10. Jasani B, Navabi H, Adams M. Ampligen: A potential toll-like 3 receptor adjuvant for immunotherapy of cancer. Vaccine 2009 27(25):3401-404.

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