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1994-10-23
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TITLE:AMPHOTERICIN B - AN INTRODUCTION
PALETTE:0223 0788 099b 09a9
AMPHOTERICIN B
by A.F.
Introduction
The Amphotericin B is a polyene antibiotic commonly used in clinical treatments
of systemic fungal infections. Isolated from a strain of Streptomyces nodosus
in 1956, it has been largely studied for years since its wide spectrum of
activity against yeasts (Candida Albicans, Cryptococcus neoformans), dimorphic
fungi (Histoplasma capsulatum, Coccidioides immitis), filamentous fungi
(Aspergillus) and even some protozoa (Leishmania and Naegleria) is accompanied
by severe secondary effects (anaphylaxis, thrombocytopenia, flushing, general
pain, convulsions, chills, fever, phlebitis, headache, anemia, anorexia and
decreased renal function) (Gallis et al., 1990, for review). The research for
AmB derivatives and delivery systems with less toxicity for host cells gains a
particular importance in relation to the increasing diffusion of
immunodepression cases, due to anti-cancer chemotherapies, corticotherapies and
AIDS.
The most evident effect of AmB is the modification of the cellular membrane
permeability which cause an alteration of the cell's ionic equilibrium and the
relative osmotic unbalance.
The mechanism of action of the molecule, anyway, is quite complex and still not
perfectly understood; a better comprehension of its dynamics may lead to
improved theraupetic results.
Chemical and physical properties.
The Amphotericin B belongs to the family of macrolide polienic antibiotics, it's
structure was completely elucidated by Mechlinski et coll. in 1970. It's
formed by a 37 Carbonium atoms lactone ring with two sides featuring different
chemical properties.
The first one is a nonpolar conjugated eptaene (seven conjugated double bounds
in trans conformation), which gives to the molecule a rigid structure and a
strong absorbtion spectrum useful for spectroscopic studies, the second one is
polar, having various hydroxyl groups, a keto and a mycosamine ring.
The molecule therefore has a amphiphilic nature and it's poorly soluble in water
for the presence of the hydrophobic region and for its zwitterionic character.
The presence of a COOH and a NH2 group gives it the characteristic amphoteric
property.
AmB is a yellow powder, soluble in strongly polar organic solvents
(dimethylsulfoxide (DMSO), methanol), it is commercially available, under the
name of Fungizone (R), associated with desoxycholate of sodium which permits a
better solubility in water.
In aqueous solution AmB exists in three mixed forms: monomers, soluble
auto-associated oligomers and insoluble aggregates of oligomers (Vertut-Croquin
et al., 1983). At low concentrations (below 5.10-8 M) it is found only in
monomeric form, at higher concentrations (between 5.10-8 M and 10-6 M) there
appear also auto-associated soluble oligomers.
The presence of insoluble aggregates at concentrations higher than 10-6 M may
have significant theraupetic influences whose importance is under investigation.
AmB in water is unstable at light, oxygen, strong temperatures and extreme pHs.
Being a polyunsatured molecule it tends to autoxidize in the presence of air
forming free radicals ad other active chemical species and losing its efficency
(Lamy-Freund et al., 1984).
Biological properties
The effects of Amphotericin B on fungal or animal cells are various. It affects
the membrane permeability for monovalent and in bivalent cations (Hartsel et
al., 1993, for review), it inhibits the action of membrane enzimes like the
proton ATPase in Candida Albicans (Solov'eva et al. 1976) or the Na+/K+ ATPase
in red blood cells and it peroxides the insature lipids (Sokol-Anderson et al.,
1986).
The membrane permeabilization, obtained with a process thought to involve the
formation of transmembrane pores, takes place generally at lower concentrations
and at faster rates. The first noticeable effect of the antibiotic incubation,
in fact, is the promotion of a ionic unbalance inside the cell, especially for
the monovalent ions Na+ and K+.
AmB has a much higher affinity for ergosterol (they bind with a 1:1
stechiometry) than for cholesterol. This explains the selectivity of the drug,
considered that animal cells membranes lack ergosterol, which is instead present
in fungal plasmalemma.
The permeabilization in animal membranes takes place only at concentrations
where AmB is found in soluble auto-associated oligomers, this suggests that the
formation of pores in ergosterol-lacking membranes may follow a different
process.
In fungal cells AmB molecules may associate with ergosterol and form pores whose
structure and dimensions have alse been described with various models (Hartsel
et al., 1993; Bolard et al., 1991). In mammal cells AmB may form a different
kind of pores, composed only by AmB molecules and possible only at high AmB
concentrations.
AmB molecules tend to autoxydate, the relative products (hydrogen peroxide
(H2O2), singlet oxygen (1O2), hydroxyl radical (OH.) and anion superoxide (O2-))
induce a peroxydation of the lipidic bilayer which can more easily be destroyed
by the osmotic unbalance caused by the earlier membrane permeabilization.
The inhibition of certain membrane enzymes seems not to be due to a direct
action of the molecule but to its modifications of the membrane structure.
Generally the cases of drug resistance in fungal cells are not a teraupetic
problem. They tend to occur relatively rarely and, furthermore, resistant cells
are usually weaker and less pathogen since the modifications in the membrane
(typically manifested as lack of ergosterol), which permits resistance to the
drug, are quite radical and often compromise the same cell's pathogeny.
Current researches are addressed towards the following directions:
- study of sinergic effects of AmB and other antifungals. AmB's membrane
permeabilization may facilitate the uptake of other agents (Brajtburg, 1992);
- discovery of AmB derivatives with better selectivity and less side effects;
- definition of new vectors which may increase the dose of AmB delivered without
raising its toxicity for the host organism. The formulation of micro
liposomial spheres (liposomes) containing the AmB to deliver is presently the
most studied and promising possibility;
- creation of AmB derivatives with feeble tendence to auto-association which may
decrease the drug toxicity;
- influence of low doses of AmB on cells of the immune system. It seems that
AmB enhance tumoricidal and microbicidal actions of macrophages (Chapman et
al, 1978; Bistoni et al, 1985) and monocytes (Martin et al, 1994).