Structure-Activity Relationship

Initial understanding of streptomycin struc- ture-activity relationships can be gleaned from comparing the activities of the compo- nent rings in groupings. Rings I and alone are sufficient for antibiotic activity,' but rings and (streptobiosamine) together are in- active; this in part explains the high tolerance of ring for modifications

Figure 6.2 highlights permissible and permissible alterations to streptomycin and analogs. Within the streptamine ring, any change to the guanidino group at C3 abolishes all activity (38-40). However, methylation of the guanidino group or substitution of that group with chemically similar moieties is usu- ally tolerated. Phosphorylation or adenylation

of the hydroxyl group at C6 abolishes activity (41); removal of that group greatly reduces ac- tivity (42). The consequences of modifying the hydroxyl group at either C2 or C5 are

Inspection of the crystal structure and sequence comparisons of se- quences from numerous organisms generally explain these observations (10). The C3

group, which cannot be altered, forms a bifurcated hydrogen bond with 0 2 ' and 0 3 ' of a universally conserved nucleotide. Methylation of the C3 guanidine would result in the disruption of one of these hydrogen bonds. Other modifications would similarly disrupt the multiple hydrogen bonds involving

the C3 guanidino group.

The guanidino group forms a salt bridge ate of When this is replaced with a carbamoyl oup, as in bluensomycin, an analogous

ction can take place and is therefore

; bluensomycin is about 10-fold streptomycin for most

mycin-sensitive organisms. Methylation of

'Unless otherwise stated, activity refers to the abil- ity of an agent to stop bacterial growth in Im- portant measurements such as binding affinity for were rarely carried out. Therefore, in general, theobserved activity (or lack of activity) is assumed

to result from a mechanism of action identical to

that of streptomycin. Examples described in the text illustrate that such assumptions are not always

the guanidino group is also tolerated and

the crystal structure sup-

ports this, because a salt bridge to the phos- phate of can be retained. Synthetic an- alogs in which the guanidino group has been replaced with

droxybutyryl] or

droxybutyryl] have been evaluated (38). These compounds were tested on a variety of organ- isms, some of which were resistant to strepto- mycin. In all cases, the reported activity was lower than for dihydrostreptomycin (which has activity similar to streptomycin) and sim- ilar to or somewhat lower than for

Replacement of the guanidino group with a 2-imidazolin-2-ylamino substituent was inactive Overall, however, most of the guanidino modifications investigated are tolerated.

Modifications of the C6 hydroxyl group are poorly tolerated. The 6-deoxydihydrostrepto- mycin analog was synthesized and tested in hopes of circumventing the action of C6- OH modifying enzymes found in some streptomy- cin-resistant bacteria, but this compound was only as active as dihydrostreptomycin (42). Phosphorylation, adenylation, or re- moval of the C6 hydroxyl group causes a strong reduction in activity; the reason is ap-

parent from the structure,

in which this hydroxyl group forms hydrogen bonds with of the ribosomal protein S12 and the phosphate of These bonds clearly must be maintained for full activity.

Although no analogs of streptomycin exist in which the C5 position is altered, the struc- ture of the complex suggests that very few, if any, modifications would be tolerated, because that hydroxyl group is also involved in a hy- drogen bond with of S12. Similar re- strictions are not expected for C2 modifica- tion, as there are no direct contacts between

its hydroxyl group and the ribosome.

The site most often exploited for strepto- mycin modification is the 3' aldehyde moiety of ring the streptose ring. The first

synthetic analog of streptomycin,

streptomycin, contained an alcohol in place of the streptose aldehyde (43). This analog was later isolated from fermentation sources (Streptomyces (44). Figure il- lustrates many of the aldehyde conversions

Therapeutic Agents Acting on RNA Targets Streptomycin CHO H H Dihydro- H H Streptomycin 5'-hydroxy- CHO H H Streptomycin CHO H H H Streptomycin Mannosido- CHO H mycin

Dimannosido- CHO H a-D-mannose-

streptomycin

Bluensomycin CHO H H

Ashimycin A CHO H Ashirnose

Ashimycin B CHO a-D-mannose-

= 5'-hydroxystreptomycin lacking ring

Figure 6.1. The structures of streptomycin and several natural analogs of streptomycin.

which have been synthesized and tested. The aldehyde oxygen forms a hydrogen bond a phosphate oxygen of

Conversion of the aldehyde to an acid or reduction to a methyl abolishes all (45, Numerous groups have investigated con- version of the aldehyde to its derivative and a variety of alkylamine derivatives, with surprising results The amino deriva-

tive and short-chain alkylamine derivatives active up to the hexylamine analog. However, activity diminished increasing length; the hexylamine derivative is only about as active as

mycin. The derivative was not in- vestigated. Longer (octyl and above) were nearly as active as

Adenylation Phosphorylation

All modifications

Figure 6.2. Molecular interactions between streptomycin and coli numbering, top) with various modifications tested for activity (bottom). Dashed lines indicate possible hydrogen bonds (some of which are salt bridges when suitably reinforced with favorable electrostatic potentials). Arrows point to permissible modifications; arrows with an X point to non-permissible modifications. (a) Ring I. Ring Dashed arrow points to modifications that results in compounds that are active by an unknown mechanism. Ring

Therapeutic Agents Acting on RNA Targets

a-D-mannose (=DM)

2"'-carboxy-xylo-furanose (ashimose)

Removal Figure 6.2. (Continued.)

up to consider the mechanism of action of Another alkylamino derivative tested was a conjugate of streptomycin and isoniazid, an- ggested that the alkylamine an- other prominent anti-tuberculosis drug. This compound termed streptohydrazid, was syn- thesized and found to be at least as active as combined therapy using both streptomycin

t

ribosomally unrelated mechanism; the and isoniazid (51). Streptohydrazid was tested long before the mechanism of action of strep- tomycin was known (the mechanism of

Therapeutic Agents Acting on RNA Targets

azid is not fully understood); it was reasoned that a conjugate of the two might act synergis- tically. The mechanism by which

drazid works is not known, but presumably it has streptomycin-like function, isoniazid-like function, or some combination of the two.

Correlation of the crystal

structure with the various streptomycin ana- logs that involve aldehyde modification sug- gests that only a hydrogen bond between the aldehyde oxygen and a protonated phosphate oxygen (or a salt bridge for the amino deriva- tives) must be maintained. Reduction in bind- ing only occurs when the modification be- comes too large to be accommodated within the binding pocket. The only exception to this is when streptomycin is oxidized to streptomy- cinic acid. Although the possibility of forming the required hydrogen bond exists, the analog is inactive, presumably because of the electro- statically unfavorable close approach of the carboxylic acid to a phosphate that would oc- cur on binding.

Several active natural streptomycin ana- logs, such as 5'-hydroxystreptomycin and are hydroxylated at (34, 52). Semisynthetic derivatives of this position are absent. A cursory inspection of the streptomy- structure suggests that the methyl group at contributes little to ribosome binding, suggesting that modifications at this position might be tolerated.

Ring (glucosamine) makes two direct contacts with neither of which are essen- tial to activity (37, 52). Indeed, all of ring can be dispensed with and cause only modest reductions in activity. However, deleterious modifications to this ring are possible. Some streptomycin-resistant bacteria harbor genes that encode proteins that either

late or adenylate the C 3 hydroxyl group of streptomycin (41). Two semisynthetic ana- logs, 3-epidihydrostreptomycin and

dihydrostreptomycin, were synthesized to cir- cumvent common streptomycin resistance (53, 54). The logic is sound: both analogs should retain streptomycin-like binding affin- ity, yet not be substrates for inactivating en- zymes. As expected, these analogs worked well against common bacterial strains and better than streptomycin against many streptomycin

resistant strains. Yet, for reasons that are not clear, they never reached clinical status.

Some glycosylations have been observed

at

C 4 in natural streptomycin analogs, yielding somewhat less active antibiotics, yet remain active against bacteria that express enzymes that phosphylate and adenylate the OH. A limited number of natural modifications oc- cur at none of which abolish activity.

5 AND TYPE

Shortly after the discovery of streptomycin,

members of the and

oxystreptamine aminoglycoside antibiotics were found in fermentation products of

nomycetes (Fig. 6.3) (Although many antibiotics contain aminoglycoside rings and are often referred to as aminoglycosides, the general term "aminoglycoside" will be used here to specifically refer to the and the and will be abbreviated AG.) Neomycin B is the prototypical AG and is composed of four rings, because it is the most effective agent in this class. The

micins are the most commonly used

and are composed of only three rings. The subclass constitutes the majority of the clinically useful agents. The two sub- classes have their first two rings in common and work by the identical mechanism of bind- ing to the decoding A-site of thus causing misreading of (57). Biochemical prob- ing has firmly established the binding of these agents to the major groove of an asymmetric loop composed in part of several absolutely conserved nucleotides (19). Other poorly con- served nucleotides within this loop also form part of the AG binding pocket and provide the basis for organismal specificity

The structure of paromomycin, a close an- alog of neomycin B, in complex with

has been solved multiple times, once by NMR

and twice by X-ray crystallography (10, 13, 59). These structures clearly describe the im- portant modes of binding for the

subclass. NMR was also used to solve the structure of C l a in complex with its target RNA sequence (60). The orientation of binding is the same-rings I and from

In document 184110582 Burger s Medicinal Chemistry and Drug Discovery Volume 2 PDF (Page 179-185)