AMPHIPHILIC POLYMERS DESIGNED FOR BIOMEDICAL APPLICATIONS
4.4 BIOMEDICAL APPLICATIONS OF AMPHIPHILIC POLYMERS Amphiphilic polymers have their own (intrinsic) biological activity, for
4.4.1 POLYMERS WITH INTRINSIC BIOACTIVITY .1 ANTIMICROBIAL AGENTS
4.4.1.2 HYPOCHOLESTEREMIC DRUGS
Cholesterol, a vital component of cell membranes, is transported in blood- stream by lipoproteins, is metabolized in liver to bile acids and is excreted in bile inside bile acid micelles or phospholipid vesicles. Excess of plasma total cholesterol and low-density lipoprotein cholesterol (LDL-CH) leads to accumulation of cholesterol in blood vessels (atherosclerotic plaque), followed by the obstruction of blood vessels (atherosclerosis), which increases the risk for the development of coronary heart disease (CHD) (Grundy, 1998). Clinical therapies developed for plasma CH level reduc- tion use different medication, such as statins, niacin, and bile acid seques- trants (BAS). Statins are the first-line agents for the treatment of primary hypercholesterolemia and fenofibrate is used in the treatment of mixed hyperlipidemia, but they are associated with serious systemic adverse effects (Bays and Goldberg, 2007), therefore, other drugs such as BAS used alone or as adjuvants in combined therapies can improve the results and reduce side effects (Robinson and Keating, 2007; Wattset al., 2016).
4.4.1.2.1 Bile Acid Sequestrants
BAS are orally administered insoluble cationic polymeric resins able to selectively bind negatively charged bile acids in the small intestine and eliminate them with feces. In order to re-establish the bile acid pool in the gall-bladder, new CH amounts will be metabolized in liver, reducing, therefore, the serum CH levels (Shepherd et al., 1980; Bays and Goldberg, 2007). Cholestyramine (an anion-exchange resin that contains benzyl trimetyl ammonium chloride groups attached to a styrene–divinylben- zene cross-linked copolymer (Shepherd et al., 1980)), and Colestipol (a cross-linked epichlorohydrin-tetraethylenepentamine resin (Class, 1991)) were for long time the only BAS in clinical use. Some inconvenience in the administration (high doses required, unpleasant taste and smell, side effects, such as nausea, abdominal discomfort, indigestion, and constipa- tion) determined a decline in the use of these drugs, but also prompted the development of more biocompatible and compliant BAS, for example, DMP-504, SK & F 97426-A, and Colesevelam hydrochloride, the last one being now commercially available in USA (Dahl et al., 2005; Alberto et al., 2009). More recent research was focused on the design of BAS
with improved selectivity for bile acids, increased binding capacity, and stability (Wang et al., 2016; Polomoscaniket al., 2012; Zhu et al., 2015;
Mendonçaet al., 2016; Lopez-Jaramilloet al., 2015), mainly by creating new chemical structures with an appropriate balance between cationic and hydrophobic groups, in order to obtain a synergistic effect of electrostatic and hydrophobic interactions between BAS and bile acids. Besides, the use of natural polymers, such as polysaccharides can significantly improve the BAS biocompatibility.
In the attempt to develop new BASs with a superior binding capacity, high selectivity for bile acids over other anions and a better biocompat- ibility, Nichifor and co-workers focused their research on aminated cross- linked polysaccharides and studied in detail the influence of the nature of polysaccharide support (dextran, pullulan, and microcrystalline cellulose), its swelling porosity and the chemical structure of amino groups (tertiary ammine versus quaternary ammonium group), on the in vitro bile acid binding capacity and affinity (Nichifor et al., 1998, 2000). Further, they studied the effect of the presence of hydrophobic segments in the structure of cationic groups of D40-QR(m)-ECH derivatives. The increase of the length of substituent R from C2 to C12 determined a significant increase in complex BAS/bile acid stability, both in the absence and presence of small electrolytes with competing anions (Nichifor et al., 2001a, 2001b).
In order to combine a high charge density with an appropriate ratio hydro- philic/hydrophobic segments, bipolar amphiphilic polymers D40-QR1(m) R2(p)-ECH were prepared and its in vitro evaluation showed that amphi- philic dextran-based BAS had much higher binding affinity for bile acid than Cholestyramine. In vivo administration of a hydrophilic dextran resin (D40-QC2(60)-ECH), an amphiphilic resin (D40-QC2(40)C12(20)-ECH) and cholestyramine to normolipemic rats (Trinca et al., 2007) demon- strated a better hypolipemic effect of the amphiphilic resin (Fig. 4.8) in comparison with hydrophilic resin and Cholestyramine. D40-QC2(40) C12(20)-ECH determined a significant decrease in total CH, low and very low-density lipoprotein (LDL and VLDL), and triglycerides, without modification of liver, pancreas, or GIT functions. The most important effect of the amphiphilic dextran resin was the increase in the ratio anti- atherogenic high-density lipoprotein (HDL)/atherogenic low-density lipo- proteins (LDL + VLDL) from 1.1 (control group) to 2.4. A higher ratio is more efficient in preventing the occurrence of atheromatous lesions than the level of each single lipid fraction.
FIGURE 4.8 Decrease of lipid level (% of control) after administration of Cholestyramine (white columns), D40-QC2(40)C12(20)-ECH (grey column) and D40-QC2(60)-ECH (black columns) to normolipemic rats (21 days and 1.2 g/kg/day).
4.4.1.2.2 Cholesterol Solubilizers
Besides its contribution to heart diseases, excess of CH is also the main risk factor for gallstone occurrence and growth due to CH crystallization.
Under normal conditions, the process is inhibited by CH dissolution in bile salt micelles, therefore, the intake of bile salts was perceived as a basis for nonsurgical therapy (Bhat et al., 2006). Despite good results obtained with chenodeoxycholic acid and ursodeoxycholic acid, clinical use of such a treatment is limited due to bile acid poor efficacy, toxic side effects, and long time required (months to years) (Konikoff, 2003; Hofmann and Hagey, 2014; Abendan and Swift, 2013), being recommended for prevention or in special clinical situation when surgery is not possible (Konikkof, 2003).
By analogy to unbound bile acid micelles, hydrophobic microdomains formed in aqueous solutions of dextran derivatives DM-BA(m) by asso- ciation of their BA pendent moieties led as to supposition that these aggre- gates could trap CH molecules inside their hydrophobic microdomains and increase thus CH solubility. The results obtained with different bile acid modified dextrans (Fig. 4.9) proved this hypothesis. As expected, the
amount of solubilized CH increased with polymer concentration, but the variation profile depended on the bound BA. The amount of solubilized CH increased almost linearly with DCA containing polymers concentra- tion, but an exponential increase was observed in the presence of DM-CA polymers. A similar behavior was observed for solubilization of CH by corresponding bile acid salts (Nagadome et al., 1995) and was related to the variation of micelle number and size with each bile acid concentration.
This similarity between the behavior of the BA-modified polymer and the free BA might indicate a similar arrangement of BA aggregates, irrespec- tive of their bound or unbound state. The increase in the dextran molar mass enhanced CH solubilization, what can be explained by the presence of a higher number of bile acid bound per polymer chain, resulting in a higher number, or size of hydrophobic microdomains/chain. Water solu- bility of CH is very low (3 × 10−8 g/mL, 0.007 mM) (Saad and Higuchi, 1965), and DM-BA polymers were able to increase this solubility up to 60 times. A direct comparison between free and bound bile acid ability to solubilize CH is difficult, but the molar ratio BA:solubilized CH is much lower in case of polymer. For example, NaCDCA:CH = 20:1 (in 2.5 mM bile salt), NaCA:CH = 63:1 (in 50 mM bile salt solution) (Mukhopadhyay
FIGURE 4.9 Variation of the amount of solubilized cholesterol with polymer concentration (expressed as bound bile acid concentration). Polymers are DM-BA(3.6), where M = 40 kDa (squares and triangles) or 200 kDa (circle) and BA is CA (triangles and circle) or DCA (squares).
and Maitra, 2004), and CA:CH = 10 (in D200-CA (3.6) solution containing 4 mM bound CA).
The capacity of polymers DM-BA to solubilize CH can find applica- tion not only in gallstone dissolution, but also in different domains, such as food or cosmetic industries.