Antibiotics - Prescription Medications

Antibiotics are among the most frequently prescribed medications in modern medicine. The first antibiotic was penicillin, discovered accidentally from a mold culture. Today, over 100 different antibiotics are available to health care professionals to cure minor discomforts as well as life-threatening infections.

Antibiotics are powerful drugs used for treating many serious diseases. They work by killing bacteria or preventing their growth. Most infections result from either bacteria or viruses. Bacteria are responsible for: most ear infections; some sinus infections; urinary tract infections. Viruses are responsible for: colds; influenza; sore throats; coughs. Most bacterial infections get better by themselves and do not require treatment. Infections such as bladder infections, sexually transmitted diseases, boils, and ear infections often require treatment with an antibiotic.

There are many different types of antibiotics and each can be used to treat a different kind of infection. Although there are several classification schemes for antibiotics, based on bacterial spectrum or type of activity (bactericidal vs. bacteriostatic), the most useful is based on chemical structure. Antibiotics within a structural class will generally show similar patterns of effectiveness, toxicity, and allergic potential.

The main classes of antibiotics include:

• Aminoglycosides
• Cephalosporins
• Fluoroquinolones
• Macrolides
• Penicillins
• Sulfonamides
• Tetracyclines

Cephalosporins

Cephalosporins are used in the treatment of infections caused by bacteria. They work by killing bacteria or preventing their growth. Cephalosporins work the same way as penicillins: they interfere with the peptidoglycan synthesis of the bacterial wall by inhibiting the final transpeptidation needed for the cross-links. This effect is bactericidal. Cephalosporins are grouped into "generations" by their antimicrobial properties. The first cephalosporins were designated first generation while later, more extended spectrum cephalosporins were classified as second generation cephalosporins. Currently, three generations of cephalosporins are recognized and a fourth has been proposed. Significantly, each newer generation of cephalosporins has greater gram negative antimicrobial properties than the preceding generation. Conversely, the "older" generations of cephalosporins have greater gram positive (staphylococcus and streptococcus) coverage than the "newer" generations.

• First generation Cephalosporins
• Cefadroxil (Duricef)
• Cephalexin (Keflex)
• Cephradine (Velosef)
• Second generation Cephalosporins
• Cefaclor (Ceclor)
• Cefuroxime Axtel (Ceftin)
• Cefprozil (Cefzil)
• Loracarbef (Lorabid)
• Third generation Cephalosporins
• Cefixime (Suprax)
• Cefpodoxime proxetil (Vantin)
• Cefdinir (Omnicef)

Fluoroquinolones

Fluoroquinolones were first introduced in 1986, but they are really modified Quinolones, a class of antibiotics, whose accidental discovery occurred in the early 1960's. Fluoroquinolone antibiotics are highly potent and considered relatively safe. The fluoroquinolones (fluoridated quinolones) are broad-spectrum bactericidal agents, that inhibit DNA gyrase, prevent DNA supercoiling and DNA synthesis. Fluoroquinolones are useful in pneumonia, urinary tract infections, bacterial diarrhea, and skin and soft tissue infection. These agents are metabolized in the liver and excreted in the urine. Fluoroquinolones are not indicated for use in infants or children, due to the potential of damaging cartilage and musculoskeletal effects (bone development problems).

• First Generation. The first-generation agents include cinoxacin and nalidixic acid, which are the oldest and least often used quinolones. These drugs had poor systemic distribution and limited activity and were used primarily for gram-negative urinary tract infections.
• Second Generation. The second-generation fluoroquinolones (ciprofloxacin, enoxacin, norfloxacin and ofloxacin.) have increased gram-negative activity, as well as some gram-positive and atypical pathogen coverage. Compared with first-generation quinolones, these drugs have broader clinical applications in the treatment of complicated urinary tract infections and pyelonephritis, sexually transmitted diseases, selected pneumonias and skin infections.
• Third Generation. The third-generation fluoroquinolones (levofloxacin, gatifloxacin, sparfloxacin) are separated into a third class because of their expanded activity against gram-positive organisms, particularly penicillin-sensitive and penicillin-resistant S. pneumoniae, and atypical pathogens.
• Fourth Generation. The fourth-generation fluoroquinolones (trovafloxacin) add significant antimicrobial activity against anaerobes while maintaining the gram-positive and gram-negative activity of the third-generation drugs.

Macrolides

The macrolide antibiotics are derived from Streptomyces bacteria, and got their name because they all have a macrocyclic lactone chemical structure. Erythromycin, the prototype of this class, has a spectrum and use similar to penicillin. Newer members of the group, azithromycin (Zithromax) and clarithyromycin, are particularly useful for their high level of lung penetration. Macrolide antibiotics are used to treat respiratory infections, such as pharyngitis, sinusitis, and bronchitis, genital, gastrointestinal tract, soft tissue, skin infections caused by susceptible strains of specific bacteria.

The mechanism of action of the macrolides is mainly bacteriostatic (inhibition the growth and reproduction of bacteria), but can also be bactericidal in high concentrations. Macrolides tend to accumulate within leukocytes, and are therefore actually transported into the site of infection.

Clarithromycin and azithromycin have similar spectrum to erythromycin but increased activity against Hemophilus, Mycobacterium avium intracellulare, and toxoplasma. Azithromycin has increased gram-negative activity, while clarithromycin has increased gram-positive activity.

Penicillins

The penicillins are the oldest class of antibiotics, and have a common chemical structure which they share with the cephalopsorins. Aminopenicillins such as ampicillin and amoxicillin have an extended spectrum of action compared with the natural penicillins; extended spectrum penicillins are effective against a wider range of bacteria.

• Natural Penicillins (Penicillin G, Procaine, Penicillin V, Benzathine). The natural penicillins were the first agents in the penicillin family to be introduced for clinical use. The natural penicillins are based on the original penicillin-G structure.
• Penicillinase-Resistant Penicillins (Cloxacillin, Dicloxacillin, Methicillin, Nafcillin, Oxacillin). Methicillin was the first member of this group, followed by oxacillin, nafcillin, cloxacillin and dicloxacillin. The penicillinase-resistant penicillins have a more narrow spectrum of activity than the natural penicillins. Penicillinase-resistant penicillins are primarily indicated for the treatment of skin and soft tissue infections.
• Aminopenicillins (also called the “broad-spectrum”) (Ampicillin, Amoxicillin, Bacampicillin). The aminopenicillins were the first penicillins discovered to be active against gram-negative bacteria (such as E. coli and H. influenzae). Aminopenicillins are acid-resistant so administered orally. Orally administered amoxicillin and ampicillin are used primarily to treat mild infections such as otitis media, sinusitis, bronchitis, urinary tract infections and bacterial diarrhea.
• Extended Spectrum Penicillins. Extended Spectrum Penicillins include both alpha-carboxypenicillins (carbenicillin and ticarcillin) and acylaminopenicillins (piperacillin, azlocillin, and mezlocillin). These agents have similar spectrums of activity as the aminopenicillins but with additional activity against several gram negative organisms of the family Enterobacteriaceae, including many strains of Pseudomonas aeruginosa.

Tetracyclines

Tetracyclines got their name because they share a chemical structure that has four rings. They are derived from a species of Streptomyces bacteria. Broad spectrum bacteriostatic agents, the tetracyclines may be effective against a wide variety of microorganisms, including rickettsia and amebic parasites. The tetracyclines are primarily bacteriostatic and are thought to exert their antimicrobial effect by the inhibition of protein synthesis. Antibiotics in tetracycline group are: demeclocycline, doxycycline, minocycline, oxytetracycline, tetracycline.