The blood-brain barrier (abbreviated BBB) is composed of endothelial cells packed tightly in brain capillaries that more greatly restrict passage of substances from the bloodstream than do endothelial cells in capillaries elsewhere in the body.
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| Composition of blood brain barrier |
Processes from astrocytes surround the epithelial cells of the BBB providing biochemical support to the epithelial cells.
- The BBB should not be confused with the blood-cerebrospinal fluid barrier, a function of the choroid plexus.
- The blood-brain barrier protects the brain from the many chemicals flowing around the body.
- Many bodily functions are controlled by hormones, which are detected by receptors on the plasma membranes of targeted cells throughout the body.
- The secretion of many hormones are controlled by the brain.
- The blood-brain barrier is an effective way to protect the brain from common infections.
- Thus infections of the brain are very rare; however, as antibodies are too large to cross the blood-brain barrier, when infections of the brain do occur they can be very serious and difficult to treat.
- By three methods drugs cross the BBB ;
1. active transfer – energy source required
2. passive transfer – according to gradient
3. facilitative transfer – non energy dependent
- Lipophilicity (‘fat-liking’) is the most important physical property of a drug in relation to its absorption, distribution, potency and elimination.
- Molecular size is one of the most important factors affecting biological activity, but it’s also one of the most difficult to measure.
- Plasma protein binding also strongly influence entry of drug into BBB, only plasma protein unbound can freely penetrate.
Veterinary drugs that cross blood brain barrier:
1. Cephalosporins
Most cephalosporins generally have excellent coverage against gram-positive pathogens, and most
second- and third-generation cephalosporins have good gram-negative activity.Cephalosporins tend to have low lipophilicity and subsequently have poor BBB penetration. However, third-generation cephalosporins are more lipophilic, which allows them to more easily cross inflamed meninges.
The mechanism of action is time-dependent and requires drug maintenance levels to be above the minimum inhibitory concentration for prolonged periods of time. The low toxicity of these drugs can be beneficial for cases in which high doses may need to be administered for longer periods.
2. Fluoroquinolones
Fluoroquinolones are moderately lipophilic, have low-to-moderate protein-binding capacity, and are of low molecular weight. All of these characteristics allow for good penetration into the CNS. Fluoroquinolones are typically used for treating gram-negative aerobic infections and also have bactericidal activity. Enrofloxacin is common drug of this group.
3. Metronidazole
Primarily used to treat anaerobic infections, metronidazole has bactericidal activity and readily penetrates the CNS. Drug levels in the CNS are high, reaching ≈90% of what is reached in the serum.
4. Trimethoprim–Sulfonamide Combinations
Trimethoprim–sulfonamide is a drug combination that reaches therapeutic levels in the CNS, regardless of whether the BBB is inflamed or intact.
5. Ampicillin
Penicillins are a group of antibiotics that have low CNS penetration.
Although concentrations increase as meningeal inflammation occurs, an efflux pump actively removes penicillins from the CNS, which results in lower concentrations.
6. Chloramphenicols
chloramphenicol crosses the blood brain barrier, it is indicated in infections of the central nervous system caused by susceptible organisms.
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