Half lives, biological

The body excretes tritium with a biological half-life of 8—14 d (10.5 d average) (75), which can be reduced significantly with forced fluid intake. For humans, the estimated maximum permissible total body burden is 37 MBq (1 mCi). The median lethal dose (LD q) of tritium assimilated by the body is estimated to be 370 GBq (10 Ci). Higher doses can be tolerated with forced fluid intake to reduce the biological half-life. 

Biological half-life The time required for one-half of the material accumulated in a tissue to be removed. 

These small-molecule thiols serve to transfer NO from erythrocytes to endothelial receptors, where it acts to relax vascular tension. NO itself is a reactive free-radical compound whose biological half-life is very short (1-5 sec). S-nitrosoglutathione has a half-life of several hours. 

Artemisinin and its derivatives, artesunate and arthemether, kill both asexual and sexual blood stages (Fig. 2). However, artemisinins are quickly eliminated from the body, resulting in parasite recrudescence, and are therefore combined with schizontocides that have a longer biological half-life, such as amodiaquine,... 

The biological half-life of a radioisotope is the time required for the body to excrete half of the radioisotope. The effective half-life is the time required for the amount of a radioisotope in the body to be reduced to half its original amount, as a result of both the decay of the radioisotope and its excretion. Sulfur-35 (tu2 = 87.4 d) is used in cancer research. The biological half-life of sulfur-35 in the human body is 90. d. What is the effective half-life of sulfur-35 ... 

Barium-140 (tl/2 = 12.8 d) released in the fire at the Chernobyl nuclear plant has been found in some agricultural products in the region. The biological half-life of barium-140 in the human body is 65 d. What is the effective half-life (see Exercise 17.55) of barium-140 ... 

For several reasons, this was not surprising. First of all, most of the data on the disposition of nitrosamlnes In plant parts relies on the use of compounds such as NDMA, which, by analogy to Its behavior In animals (24). might be expected to have a short biological half-life In plants It could be especially difficult to detect such compounds when the analyses are performed several weeks after applying the precursors to the plants. In addition, one might expect to find relatively little nitrosatlon of dimethylamlne In a plant, as such relatively basic amines are... 

Corticosteroids Adjunctive treatment for acute or delayed NAV Dexamethasone 12 mg IV or PO on day 1 8 mg IV or PO daily on days 2-4 2-4 hours biologic half-life of 36-54 hours... 

In the case of radioactive materials contained in living organisms, an additional consideration is made for the reduction in observed activity due to regular processes of elimination of the respective chemical or biochemical substance from the organism. This introduces a rate constant called the biological half-life (Tbioi) which is the time required for biological processes to eliminate one-half of the activity. This time is virtually the same for both stable and radioactive isotopes of any given element. 

A typical semi-log plasma concentration versus time plot is shown in Fig. 4. This figure shows that pharmacokinetic data can also be expressed in terms of a half-life, called the biological half-life, which bears the same relationship to kei as that shown in Eqs. (14) and (15). 

Example. A 250 mg dose of tetracycline was administered to a patient by rapid IV injection. The initial plasma concentration (C ) was 2.50pg/mL. After 4hours the plasma concentration was 1.89pg/mL. What is the biological half-life of tetracycline in this patient ... 

Example. The biological half-life of procaine in a patient was 35 minutes, and its volume of distribution was estimated to be 58 L. Calculate the TCR of procaine. 

Example. Sulfadiazine in a normal volunteer had a biological half-life of 16 hours and a volume of distribution of 20 L. Sixty percent of the dose was recovered as unchanged drug in urine. Calculate TCR, RCR, and MCR for sulfadiazine in this person. 

In the foregoing example, the drug was administered to a healthy subject who had normal kidney and liver function. The estimated biological half-life in this... 

Thus, the biological half-life of a drug can increase dramatically when the organs of elimination are diseased or nonfunctional it may increase to varying degrees if these organs are partially impaired. 

Example. A drug has a biologic half-life of 4 hours. Following an IV injection of 100 mg, is found to be 10 pg/mL. Calculate Cmax and Cm n if the 100-mg IV dose is repeated every 6 hour until a plasma concentration plateau is reached. 

It is interesting that the in vitro dissolution test (USP) was more sensitive to the piroxicam formulation variables than the biodata. The fast, moderate, and slow products were found bioequivalent to each other and to the lot of innovator product studied [100]. It is possible that either the formulation variables studied did not affect in vivo dissolution and/or the differences were not discernible because of the long biological half-life of piroxicam [146]. 

---