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Figure 11.17 Ternary phase diagram of complex coacervation between pRE-luciferase plasmid and chitosan at 55°C in 50 mmol dm03 Na2SO4. Sodium sulfate solution was regarded as one component, since the concentration change in the experiment range was minimal. The region to the right of line the ABC depicts the conditions under which phase separation occurs. The concentration ranges in the small grid area yield distinct particles.

Reproduced from H.-Q. Mao etal., J. Control. Release, 70, 399-421 (2001).

log P of approximately -3.5. Like DNA they clearly do not have the appropriate properties for transfer across biological membranes. They are also sensitive to nucleases and nonspecific adsorption to biological surfaces, and hence require to be formulated to achieve delivery and nuclear uptake. Chemical modification of oligonucleotides, which is outside of the scope of this book, increases the stability and activity of the molecules.

Figure 11.18 (a) Electron micrograph of naked plasmid DNA; (b) the same sample after condensation with a cationic partial dendrimer (Ramaswamy and Florence, J. Drug Det. Sci. Technol., 15, 307-311 (2005)). The size, shape, charge and stability of such complexes depend on the ionic strength of the medium, lipid composition, lipid/DNA ratio and concentration, and even on mixing procedures. Manufacturing issues include those of repro-ducibility, avoiding aggregation of the resulting colloidal suspension and lyophilisation, as discussed in R. I. Mahato et al., Cationic lipid-based gene delivery systems: pharmaceutical perspectives, Pharm. Res., 14, 853-859 (1997).

Figure 11.18 (a) Electron micrograph of naked plasmid DNA; (b) the same sample after condensation with a cationic partial dendrimer (Ramaswamy and Florence, J. Drug Det. Sci. Technol., 15, 307-311 (2005)). The size, shape, charge and stability of such complexes depend on the ionic strength of the medium, lipid composition, lipid/DNA ratio and concentration, and even on mixing procedures. Manufacturing issues include those of repro-ducibility, avoiding aggregation of the resulting colloidal suspension and lyophilisation, as discussed in R. I. Mahato et al., Cationic lipid-based gene delivery systems: pharmaceutical perspectives, Pharm. Res., 14, 853-859 (1997).

11.6 Therapeutic monoclonal antibodies33

There are now a number of monoclonal antibodies (antibodies produced from a single clone of cells) available for use in the clinic. Although they have been available since the mid 1970s, there have been problems, not least with their poor distribution and tissue penetration. They are large molecules and diffuse slowly in tissues, especially tumours or inflamed sites. The intravenous route has been most commonly used, so formulations have so far been linked to the development of stable solutions. Physical chemistry can explain in part their mechanism of action, which includes33 blocking or through steric hindrance the action of target antigens, binding to an antigen, diffusion and translocation in tissues. The literature seems sparse on these topics.

Summary

This chapter has not attempted to provide a complete overview of proteins and peptides and other complex therapeutics such as DNA, oligonucleotides and monoclonal antibodies. That would require a whole textbook in itself. It has tried to give some feeling for the pro perties of peptides and proteins and how their physical properties are dictated not only by the properties of their individual amino acids but also by the spatial arrangement of the amino acids in their polypeptide chains. Hydrophobic amino acids alternating with hydrophilic, or charged amino acids can produce tertiary and quaternary structures which are quite different from those induced by the spatial separation of hydrophobic and hydrophilic amino acids. The stability of proteins and peptides is of prime pharmaceutical significance. Maintenance of chemical and physical stability is a prerequisite for clinical use. The macromolecular and generally hydro-philic character of all but the smallest peptides makes this class of compound one which is challenging in terms of delivery for therapeutic ends. Some of the issues involved in the formulation of proteins are discussed in the chapter.

DNA and oligonucleotides provide similar challenges - large hydrophilic molecules with no propensity for movement across cell walls and prone to degradation physically and chemically.

Monoclonal oligonucleotides were deprived of their early promise by their size, adverse properties and poor diffusion characteristics, but if monoclonal antibodies are used by the intravenous route they can exert powerful effects in vivo.

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