Why C-6 Deoxy Tetracyclines Are More Stable

Stable chelate complexes are formed by the tetracyclines with many metals, including calcium, magnesium, and iron. Such chelates are usually very insoluble in water, accounting for the impaired absorption of most (if not all) tetracy-clines in the presence of milk; calcium-, magnesium-, and aluminum-containing antacids; and iron salts. Soluble alka-linizers, such as sodium bicarbonate, also decrease the GI absorption of the tetracyclines.155 Deprotonation of tetracy-clines to more ionic species and the observed instability of these products in alkaline solutions may account for this observation. The affinity of tetracyclines for calcium causes them to be incorporated into newly forming bones and teeth as tetracycline-calcium orthophosphate complexes. Deposits of these antibiotics in teeth cause a yellow discoloration that darkens (a photochemical reaction) over time. Tetracyclines are distributed into the milk of lactating mothers and will cross the placental barrier into the fetus. The possible effects of these agents on the bones and teeth of the child should be considered before their use during pregnancy or in children younger than 8 years of age.

Mechanism of Action and Resistance

The strong binding properties of the tetracyclines with metal ions caused Albert156 to suggest that their antibacterial properties may be because of an ability to remove essential metal ions as chelated compounds. Elucidation of details of the mechanism of action of the tetracyclines,157 however, has defined more clearly the specific roles of magnesium ions in molecular processes affected by these antibiotics in bacteria. Tetracyclines are specific inhibitors of bacterial protein synthesis. They bind to the 30S ribosomal subunit and, thereby, prevent the binding of aminoacyl tRNA to the mRNA-ribo-some complex. Both the binding of aminoacyl tRNA and the binding of tetracyclines at the ribosomal binding site require magnesium ions.158 Tetracyclines also bind to mammalian ribosomes but with lower affinities, and they apparently do not achieve sufficient intracellular concentrations to interfere with protein synthesis. The selective toxicity of the tetracyclines toward bacteria depends strongly on the self-destructive capacity of bacterial cells to concentrate these

agents in the cell. Tetracyclines enter bacterial cells by two processes: passive diffusion and active transport. The active uptake of tetracyclines by bacterial cells is an energy-dependent process that requires adenosine triphosphate (ATP) and magnesium ions.159

Three biochemically distinct mechanisms of resistance to tetracyclines have been described in bacteria160: (a) efflux mediated by transmembrane-spanning, active-transport proteins that reduces the intracellular tetracycline concentration; (b) ribosomal protection, in which the bacterial protein synthesis apparatus is rendered resistant to the action of tetracyclines by an inducible cytoplasmic protein; and (c) enzymatic oxidation. Efflux mediated by plasmid or chromosomal protein determinants tet-A, -E, -G, -H, -K, and -L, and ribosomal protection mediated by the chromosomal protein determinants tet-M, -O, and -S are the most frequently encountered and most clinically significant resistance mechanisms for tetracyclines.

Spectrum of Activity

The tetracyclines have the broadest spectrum of activity of any known antibacterial agents. They are active against a wide range of Gram-positive and Gram-negative bacteria, spirochetes, mycoplasma, rickettsiae, and chlamydiae. Their potential indications are, therefore, numerous. Their bacterio-static action, however, is a disadvantage in the treatment of life-threatening infections such as septicemia, endocarditis, and meningitis; the aminoglycosides and/or cephalosporins usually are preferred for Gram-negative and the penicillins for Gram-positive infections. Because of incomplete absorption and their effectiveness against the natural bacterial flora of the intestine, tetracyclines may induce superinfections caused by the pathogenic yeast Candida albicans. Resistance to tetracyclines among both Gram-positive and Gram-negative bacteria is relatively common. Superinfections caused by resistant S. aureus and P. aeruginosa have resulted from the use of these agents over time. Parenteral tetracyclines may cause severe liver damage, especially when given in excessive dosage to pregnant women or to patients with impaired renal function.

Structure-Activity Relationships

The large amount of research carried out to prepare semisynthetic modifications of the tetracyclines and to obtain individual compounds by total synthesis revealed several interesting SARs. Reviews are available that discuss SARs among the tetracyclines in detail,161-163 their molecular and clinical properties,164 and their synthesis and chemical prop-erties.162,163,165,166 Only a brief review of the salient structure-activity features is presented here. All derivatives containing fewer than four rings are inactive or nearly inactive. The simplest tetracycline derivative that retains the characteristic broad-spectrum activity associated with this antibiotic class is 6-demethyl-6-deoxytetracycline. Many of the precise structural features present in this molecule must remain unmodified for derivatives to retain activity. The integrity of substituents at carbon atoms 1, 2, 3, 4, 10, 11, 11a, and 12, representing the hydrophilic "southern and eastern" faces of the molecule, cannot be violated drastically without deleterious effects on the antimicrobial properties of the resulting derivatives.

A-ring substituents can be modified only slightly without dramatic loss of antibacterial potency. The enolized tricar-bonylmethane system at C-1 to C-3 must be intact for good activity. Replacement of the amide at C-2 with other functions (e.g., aldehyde or nitrile) reduces or abolishes activity. Monoalkylation of the amide nitrogen reduces activity proportionately to the size of the alkyl group. Aminoalkylation of the amide nitrogen, accomplished by the Mannich reaction, yields derivatives that are substantially more water soluble than the parent tetracycline and are hydrolyzed to it in vivo (e.g., rolitetracycline). The dimethylamino group at the 4-position must have the a orientation: 4-epitetracyclines are very much less active than the natural isomers. Removal of the 4-dimethylamino group reduces activity even further. Activity is largely retained in the primary and N-methyl secondary amines but rapidly diminishes in the higher alkyl-amines. A cis-A/B-ring fusion with a j-hydroxyl group at C-12a is apparently also essential. Esters of the C-12a hy-droxyl group are inactive, with the exception of the formyl ester, which readily hydrolyzes in aqueous solutions. Alkylation at C-11a also leads to inactive compounds, demonstrating the importance of an enolizable j-diketone functionality at C-11 and C-12. The importance of the shape of the tetracyclic ring system is illustrated further by substantial loss in antibacterial potency resulting from epimer-ization at C-5a. Dehydrogenation to form a double bond between C-5a and C-11a markedly decreases activity, as does aromatization of ring C to form anhydrotetracyclines.

In contrast, substituents at positions 5, 5a, 6, 7, 8, and 9, representing the largely hydrophobic "northern and western" faces of the molecule, can be modified with varying degrees of success, resulting in retention and, sometimes, improvement of antibiotic activity. A 5-hydroxyl group, as in oxytetracycline and doxycycline, may influence pharmaco-kinetic properties but does not change antimicrobial activity. 5a-Epitetracyclines (prepared by total synthesis), although highly active in vitro, are unfortunately much less impressive in vivo. Acid-stable 6-deoxytetracyclines and 6-demethyl-6-deoxytetracyclines have been used to prepare various monosubstituted and disubstituted derivatives by electrophilic substitution reactions at C-7 and C-9 of the D ring. The more useful results have been achieved with the introduction of substituents at C-7. Oddly, strongly electron-withdrawing groups (e.g., chloro [lortetracycline] and nitro) and strongly electron-donating groups (e.g., dimethylamino [minocycline]) enhance activity. This unusual circumstance is reflected in QSAR studies of 7- and 9-substituted tetracyclines,162,167 which indicated a squared (parabolic) dependence on a, Hammet's electronic substituent constant, and in vitro inhibition of an E. coli strain. The effect of introducing substituents at C-8 has not been studied because this position cannot be substituted directly by classic electrophilic aromatic substitution reactions; thus, 8-substituted derivatives are available only through total synthesis.168

The most fruitful site for semisynthetic modification of the tetracyclines has been the 6-position. Neither the 6a-methyl nor the 6^-hydroxyl group is essential for antibacterial activity. In fact, doxycycline and methacycline are more active in vitro than their parent oxytetracycline against most bacterial strains. The conversion of oxytetracycline to doxycycline, which can be accomplished by reduction of methacycline,169 gives a 1:1 mixture of doxycycline and epidoxycycline (which has a ^-oriented methyl group); if the C-11a a-fluoro derivative of methacycline is used, the ^-methyl epimer is formed exclusively.170 6-Epidoxycycline is much less active than doxycycline. 6-Demethyl-6-deoxytetracycline, synthesized commercially by catalytic hydrogenolysis of the 7-chloro and 6-hydroxyl groups of 7-chloro-6-demethyltetracy-cline, obtained by fermentation of a mutant strain of Streptomyces aureofaciens,171 is slightly more potent than tetracycline. More successful from a clinical standpoint, however, is 6-demethyl-6-deoxy-7-dimethylaminotetracycline (minocycline)172 because of its activity against tetracycline-resistant bacterial strains.

6-Deoxytetracyclines also possess important chemical and pharmacokinetic advantages over their 6-oxy counterparts. Unlike the latter, they are incapable of forming anhy-drotetracyclines under acidic conditions because they cannot dehydrate at C-5a and C-6. They are also more stable in base because they do not readily undergo ^-ketone cleavage, followed by lactonization, to form isotetracyclines. Although it lacks a 6-hydroxyl group, methacycline shares the instability of the 6-oxytetracyclines in strongly acetic conditions. It suffers prototropic rearrangement to the anhydrotetracycline in acid but is stable to ^-ketone cleavage followed by lac-tonization to the isotetracycline in base. Reduction of the 6-hydroxyl group also dramatically changes the solubility properties of tetracyclines. This effect is reflected in significantly higher oil/water partition coefficients of the 6-de-oxytetracyclines than of the tetracyclines (Table 8.8).173,174 The greater lipid solubility of the 6-deoxy compounds has important pharmacokinetic consequences.162,164 Hence, doxycycline and minocycline are absorbed more completely following oral administration, exhibit higher fractions of plasma protein binding, and have higher volumes of distribution and lower renal clearance rates than the corresponding 6-oxytetracyclines.

Polar substituents (i.e., hydroxyl groups) at C-5 and C-6 decrease lipid versus water solubility of the tetracyclines. The 6-position is, however, considerably more sensitive than the 5-position to this effect. Thus, doxycycline (6-deoxy-5-oxytetracycline) has a much higher partition coefficient than either tetracycline or oxytetracycline. Nonpolar substituents (those with positive a values; see Chapter 2),

TABLE 8.8 Pharmacokinetic Properties3 of Tetracyclines

Tetracycline

Kpc Octanol/ Water pH 5.6b

Absorbed

Orally

Excreted in Feces (%)

Excreted in Urine (%)

Protein Bound

Volume of Distribution (% body weight)

Renal

Clearance Half-

Tetracycline

Kpc Octanol/ Water pH 5.6b

Absorbed

Orally

Excreted in Feces (%)

Excreted in Urine (%)

Protein Bound

Volume of Distribution (% body weight)

Renal

Clearance Half-

Tetracycline

Was this article helpful?

0 0
Healthy Sleep

Healthy Sleep

A Guide to Natural Sleep Remedies. Many of us experience the occasional night of sleeplessness without any consequences. It is when the occasional night here and there becomes a pattern of several nights in arow that you are faced with a sleeping problem. Repeated loss of sleep affects all areas of your life The physical, the mental, and theemotional. Sleep deprivation can affect your overall daily performance and may even havean effecton your personality.

Get My Free Ebook


Responses

  • iggi mehari
    Why c6 deoxy tetracyclines are more stable?
    4 years ago

Post a comment