It can also be re-absorbed in the renal tubules and gastrointestinal tract due to its high lipophillicity so has a long elimination half life, and does not accumulate in the kidneys of patients with kidney failure due to the compensatory excretion in faeces. Doxycycline–metal ion complexes are unstable at acid pH, therefore more doxycycline enters the duodenum for absorption than the earlier tetracycline compounds. In addition, food has less effect on absorption than on absorption of earlier drugs with doxycycline serum concentrations being reduced by about 20% by test meals compared with 50% for tetracycline. Mechanism of action
Doxycycline is a broad-spectrum bacteriostatic antibiotic. It inhibits the synthesis of bacterial proteins by binding to the 30S ribosomal subunit, which is only found in bacteria. This prevents the binding of transfer RNA to messenger RNA at the ribosomal subunit meaning amino acids cannot be added to polypeptide chains and new proteins cannot be made. This stops bacterial growth giving the immune system time to kill and remove the bacteria. Pharmacokinetics
The substance is almost completely absorbed from the upper part of the small intestine. It reaches highest concentrations in the blood plasma after one to two hours and has a high plasma protein binding rate of about 80–90%. Doxycycline penetrates into almost all tissues and body fluids. Very high concentrations are found in the gallbladder, liver, kidneys, lung, breast milk, bone and genitals; low ones in saliva, aqueous humour, cerebrospinal fluid (CSF), and especially in inflamed meninges.
Atypical lichen myxedematosus  is a skin condition caused by fibroblasts producing abnormally large amounts of mucopolysaccharides. : 186

See also
Lichen myxedematosus
Skin lesion

References


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