AMINOGLYCOSIDE ANTIBACTERIALS

Copyright, Purdue Research Foundation, 1996

Overview

Important group
Spectum - Gram(-) aerobes
Narrow margin of safety

Renal toxicity
Ototoxicity
Neuromuscular weakness

Polycations -- Limited membrane transfer

Members

Streptomycin
Amikacin
Gentamicin
Kanamycin
Neomycin
Tobramycin
Netilmicin
Apramycin (vet only)

Structure & Chemical Characteristics

Aminosugars

Glycosidic bonds -- hence name
Weak bases
polycations

Two "bases"

streptidine (streptomycin)
2-deoxystreptamine (most)

Source

Most -- Streptomyces spp.
Gentmicin -- Micromonospora actinomyces
Note spelling differences

Subgrouping -- little practical value

Mechanism of action

Cidal
Bind strongly to 30S ribosomal subunit
Inhibit initiation of protein synthesis
Decrease fidelity of reading mRNA

Faulty proteins
Cidal effect (?)

Most active on growing organisms

Resistance

Transport system crucial

Oxygen dependent
Missing in anaerobes

Most organisms sensitive at ribosomal level
Acquired resistance

Plasmid borne
Families of destructive enzymes

phosphorylases
acetylases
adenyl transferases

Targets blocked on some drugs

Goodman & Gilman Figure 51-1, 7th ed, 1985
Major emphasis of development
Note: Amikacin
No clear-cut pattern on grouping
One aminoglycoside not predict others

Extent of resistance

Increases with increased use
Decreases if stop use -- hospital experience

One-step mutations -- Streptomycin

Can occur rapidly
Subunit S12, position 42 of ribosome

Sensitive -- LYS
Resistant -- ASPn
Dependent -- GLUn

Pharmacokinetics

Membrane transfer

Polycationic nature

very water soluble
poorly lipid soluble
limited passive diffusion through membranes

Ziv et al, IV infusions into cows
"Aminoglycosides" rough average of gentamicin, kanamycin, and neomycin

Aminoglycoside Diffusion into Milk
Milk:Plasma Drug Concentration Ratio

Drug Predicted ratio Actual ratio

Aminoglycosides 7 0.2-0.4

Tylosin 5 1-5
Carrier mediated transport -- pinocytosis into some cells

Aminoglycoside Diffusion into Milk
Milk:Plasma Drug Concentration Ratio
Drug	Predicted ratio	Actual ratio
Aminoglycosides	7	0.2-0.4
Tylosin	5	1-5

Other pharmacokinetics

No slowly absorbed dose forms
All given as water soluble "sulfate" salts
Most IM or SC

"STICK" to tissues at injection site
DRUG RESIDUES!

PO for gastrointestinal tract action

Distribution

Broadly distributed in ECF
Poor in CNS or eye
Poor intracellular
May cross placenta
Relatively high concentrations in

kidney, cochlea, vestibular apparatus
Not therapeutically relevant
Toxicity and Residue relevant
Tissue half-lives >80 to 100 hours!
How many half-lives to get to "action level"?

Elimination

Half-life -- 2 to 3 hr normally
Feces if PO
Glomerular filtration if systemic administration
More than 90%
Concern for renal function

Use creatinine clearance or other measure
Narrow margin of safety
Therapeutic drug monitoring
Dosage adjustment schemes
Priming dose not change
Adjust

Dose
Interval -- probably best
Both

Examples -- Gentamicin in canine models

Conc of gentamicin SHOULD --

Drop below 2 mcg/mL
Exceed 5 mcg/mL
NOT exceed 12 mcg/mL

Figures

Schemes for dosage adjustment assistance

NOTE: BMS 445 - Do not print these images from computer lab, They are large and you will be extremely frustrated! Go to the library to copy them.
[pic] Graph -- Effect of decreased renal elimination
[pic] Graph -- Effect of altering dose
[pic] Graph -- Effect of altering interval
[pic] Graph -- Effect of altering dose and interval
[pic] Nomogram for Aminoglycoside Dosage Adjustment

Target concentrations

       Drug           Therapeutic        Maximum Peak       Maximum Trough   
                     Concentration       Concentration      Concentration    
                        (mcg/mL)           (mcg/mL)            (mcg/mL)      

Amikacin                 15-25                35                  5          

Gentamicin                4-10                10                  2          

Kanamycin                15-30               30-35                5          

Netilmicin                6-12                16                  2          

Streptomycin               --               20-25*                --         

Tobramycin                4-10                10                  2          

* In patients with renal damage. Peak concentrations greater than 50 mcg  
per mL are associated with increased risk of toxicity. [USPDI95,71]

Adverse effects

Dose related!!!!
Concentration x Time dependence
proximal tubular cell damage
Destruction of sensory cells in cochlea
Destruction of sensory cells in vestibular apparatus
Neuromuscular paralysis

Nephrotoxicity

Toxicity WILL occur
May not see clinically
Limit use if no therapeutic monitoring
Maximum 5 days ??
Concentration must drop below 2 mcg/mL each interval
Proximal tubular cell

Enter by pinocytosis
Rate limited
May be best to have VERY high conc less often
Cell cannot handle drug
Drug accumulates in vesicles
Cytosegresomes

Must monitor renal function if long use

proteinuria
casts
specific enzymes

Ototoxicity: cochlear and vestibular apparatus

loss of hearing, vertigo, ataxia, loss of balance
Concentration / Time dependent (AUC)
Progressive damage to sensory cells
No regeneration of lost cells
All aminoglycosides
Short term Rx humans -- 1-3% morbidity
Vestibular toxicity -- especially gentamicin
Cochlear toxicity -- especially kanamycin, amikacin, neomycin
ALL affect both, however!!

Neuromuscular paralysis

Inhibit Ca⁺⁺ into nerve on depolarization

required for exocytotic ACh release
Ca⁺⁺ injections can improve release

Weakness at doses on top end of range if any renal problem
Respiratory paralysis if use for lavage of peritoneal or pleural cavity!
Story of Purdue dog -- AG - po + IM

Clincal use

Routes

Topically
Orally (Local GI action)

e.g., newborn pigs - diarrhea

Nebulization for respiratory infections [USPDI94].

amikacin
gentamicin
tobramycin

Serious UTI
Systemic

Septicemias caused by aerobic gram-negative organisms
Newer beta-lactams have decreased use, but still mainstays
MICs

gentamicin & tobramycin -- 4-8 mcg/mL
kanamycin & amikacin -- 8-16 mcg/mL
Less toxic than gent & tob.

Alkalinization of urine

Streptomycin 20-30 times more active at pH 8 than 5.8 (BM6th,1988. p823)

Spectrum

Pasteurella, Brucella, Hemophilus, Salmonella, Klebsiella, Shigella
Mycobacterium spp. (some aminoglycosides).
Synergy with beta-lactams
Infections commonly indicated
biliary tract infections, bone and joint infections, central nervous system infections (including meningitis and ventriculitis), intra-abdominal infections (including peritonitis), pneumonia (gram-negative), septicemia, skin and soft tissue infections (including burn wound infections), and urinary tract infections (complicated, recurring). Streptomycin is used for the treatment of brucellosis, granuloma inguinale, plague, tuberculosis, and tularemia. [USPDI94]
dogs, primary topical application is for otitis externa. Must be sure ear drum is intact. If ruptured, direct access to middle ear produces ototoxicity.

Miscellaneous

Apramycin

Apramycin, used in veterinary medicine only, is contraindicated in cats. [Brander5th,1991]
Minly indicated for reatment of colibacillosis and salmonellosis in calves and pigs. Can be used p.o. or parenterally. [Brander5th,1991]

References

J. NeuroSci. 8(9):3354, 1988. Aminoglycosides on synaptosomes. Decrease voltage dependent calcium entry.
GG7th, 1985. Chapter 51
BM5th, 1988
Brander5th, 1991: Chapter 26 in Brander, G.C., D.M. Pugh, R.J. Bywater, & W.L. Jenkins. Veterinary Applied Pharmacology & Therapeutics., Bailliere Tindall, London, 5th edition, 1991.
USPDI94; USPDI95: United States Pharmacopeia Drug Information, USPC, 12601 Twinbrook Parkway, Rockville, MD 20852 (published annually with monthly updates).
Godber, L.M., Walker, R.D., Stein, G.E., Hauptman, J.G., and Derksen, F.J.: 1995. Pharmacokinetics, nephrotoxicosis, and in vitro antibacterial activity associated with single versus multiple (three times) daily gentamicin treatments in horses. Am. J. Vet. Res. 56(5):613-618. important new concepts in this paper for both medical and veterinary students. Good references, too. Also, other papers in notebook that should be added. 5/31/95.

Study Questions

Examine the structures of the aminoglycosides and predict the ease with which they cross membranes by passive diffusion. What does this imply about concentrations achievable inside of cells, in mammary gland milk cisterns, systemic circulation after oral administration, etc.?
Given that the major route of elimination of aminoglycosides from systemic circulation is by glomerular filtration and knowledge of the membrane transfer of these drugs, what is the difference in their route of elimination when administered orally versus parenterally?
Why are aminoglycosides among the main drugs for which therapeutic drug monitoring is used?
Are the aminoglycosides effective in treating CNS infections when given systemically?
What is the general antibacterial spectrum of the aminoglycosides? Are they active against anaerobes? Why or why not?
How does the concentration of aminoglycosides required for antibacterial activity compare to the concentration associated with toxicity? Compare this range in general terms (e.g. much narrower, the same, larger) to that which can be used clinically with penicillin G?
What is the mechanism of action of the aminoglycosides? Understand how irreversible inhibition of protein synthesis and decreased fidelity of synthesis which results in insertion of faulty proteins in cell membranes might result in the aminoglycosides being cidal. Note that the cidal effect may be due to the extremely tight binding of aminoglycosides to the 30S ribosomal subunit so that once inhbited, reversal is unlikely.
Which of the biochemical mechanisms is responsible for the acquired R-factor related (plasmid borne) bacterial resistance to aminoglycoside action?
Name the three major enzyme classes that inactivate the aminoglycosides.
How do the various aminoglycosides compare in susceptibility to different isozymes of these three classes of enzymes?
What uniqueness does amikacin currently hold with respect to its susceptibility to these enzymes? What is the relevance of these differences in selecting an aminoglycoside for therapy? If an organism is resistant to one aminoglycoside is it necessarily resistant to another, i.e., can you use one aminoglycoside to predict the activity of another?
What are the 4 major side effects (3 main classes) of the aminoglycosides? (hint: counting 2 side effects under ototoxicity) Is there any doubt that the aminoglycosides can cause the side effects for which they are well known? How does the concept of Area Under the Concentration x Time curve apply here?
Why is neomycin not used systemically? Note that there are no approved parenteral, systemic dose forms for neomycin.
You should be able to interpret the nomogram [in lecture notes] for adjustment of gentamicin dosage in the presence of compromised renal function. This is just an example of the many such nomograms available, some simpler and many more complicated, for adjusting aminoglycoside dosage.

Spectinomycin

Aminocyclitol (related to aminoglycosides structurally)
Source: Streptomyces spectabilis
Protein synthesis inhibitor
- Not cidal
- No misreading of mRNA
- Resistance develops rapidly
Adverse effects
- Low toxicity
- NO nephrotoxicity
- NO ototoxcity
- See other sources for additional effects
Uses
- Gram negative bacteria
- FDA approved human med -- Penicillin resistant Neisseria gonococcus
- IM humans
- Feed-based preparations for animals.
  - pneumonias - swine
  - broad spectrum G(+ / -) infections
  - coliform diarrheas - piglets
- Drinking water -- turkeys

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Gordon L. Coppoc, DVM, PhD
Professor of Veterinary Pharmacology
Head, Department of Basic Medical Sciences
School of Veterinary Medicine
Purdue University
West Lafayette, IN 47907-1246
Tel: 317-494-8633Fax: 317-494-0781
Email: coppoc@vet.purdue.edu

Last modified 1:49 PM on 4/12/96 GLC