Therapeutic Indices

The median effective and toxic doses are used to determine the therapeutic index (TI).24 The TI is calculated as the ratio of the TD o to the ED50  

It should be noted, however, that the TI is a relative term. Acetaminophen, a nonprescription analgesic, has a TI of approximately 27 (i.e., the ratio of the median toxic dose to the median effective dose equals 27). Prescription agents tend to have lower TIs. For instance, the narcotic analgesic meperidine (Demerol) 

Berkowitz BA. Basic and clinical evaluation of new dmgs. In Katzung BG, ed. Basic and Clinical Pharmacology. 9th ed. New York Lange Medical Books/McGraw Hill 2004. 

Berman A. Reducing medication errors through naming, labeling, and packaging. / Med Syst. 2004 28 9-29. 

Borgheini G. The bioequivalence and therapeutic efficacy of generic versus brand-name psychoactive drugs. Clin Ther. 2003 25 1578-1592. 

FIGURE 1.11 a) The concept of antagonism or potentiation of pharmacologic effects, b) Drugs with higher therapeutic indices are safer. TI = therapeutic index, LD50 = median lethal dose, ED50 = median effective dose. 

FIGURE 1.12 A simplified scheme for the mechanism of action of cytochrome P-450. NADP = nicotinamide-adenine dinucleotide phosphate, NADPH = the reduced form of NADP. 

The higher the therapeutic index, the safer the drug the lower the therapeutic index, the greater the possibility of toxicity. The therapeutic index for barbiturate as a class is 10, whereas the therapeutic index for cardiac glycoside as a class is 3. Because the usual therapeutic dose of cardiac glycoside is 1 mg, death may result if only 3 mg has been administered. 

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Fig. 1. Blood—drug concentration curve used to determine bioavailabiLitv and bioequivalence. C is the maximum dmg concentration in the blood and corresponds to some The AUC (shaded) represents the total amount of orally adininistered dmg the time from points A to B represents dmg onset, from points B to D, the duration MEC = minimum effective concentration MTC = minimum toxic concentration and TI = therapeutic index.

The therapeutic efficacy of a dmg is generally measured in terms of ED q or ID q which represent the concentration of dmg which produces 50% of the maximum effect or 50% of maximum inhibition. LD q represents the concentration of dmg that produces 50% fataUties in test animals. The therapeutic index is the ratio of the ED q versus LD q. Detailed descriptions of the terminology and fundamental principles of pharmacology are available (32) (see Pharmacodynamics). 

Antineoplastic Drugs. Cyclophosphamide (193) produces antineoplastic effects (see Chemotherapeutics, anticancer) via biochemical conversion to a highly reactive phosphoramide mustard (194) it is chiral owing to the tetrahedral phosphoms atom. The therapeutic index of the (3)-(-)-cyclophosphamide [50-18-0] (193) is twice that of the (+)-enantiomer due to increased antitumor activity the enantiomers are equally toxic (139). The effectiveness of the DNA intercalator dmgs adriamycin [57-22-7] (195) and daunomycin [20830-81-3] (196) is affected by changes in stereochemistry within the aglycon portions of these compounds. Inversion of the carbohydrate C-1 stereocenter provides compounds without activity. The carbohydrate C-4 epimer of adriamycin, epimbicin [56420-45-2] is as potent as its parent molecule, but is significandy less toxic (139). 

Improvements in asthma treatment include the development of more effective, safer formulations of known dmgs. The aerosol adrninistration of P2-agonists or corticosteroids results in a decrease in side effects. Also, the use of reUable sustained release formulations has revolutionized the use of oral xanthines which have a very narrow therapeutic index (see Controlled release technology). For many individuals, asthma symptoms tend to worsen at night and the inhaled bronchodilatots do not usually last through an entire night s sleep (26,27). 

Historically, the use of xanthines has been hampered by poor aqueous solubiUty, rapid but highly variable metaboHsm, and the existance of a low therapeutic index. SolubiUty problems were partially solved by the preparation of various salt forms, eg, aminophylline. However, it was since recognized that the added base in aminophylline only increases solubiUty by increasing pH and thus does not affect the rate of absorption from the gut (65). Thus, in more recent medical practice, theophylline is commonly dispensed in anhydrous form and aminophylline is only recommended for iv adrninistration. 

A.n log ue Synthesis. Two notable examples, in which analogues have greater therapeutic indexes than the parent dmgs, have been identified in Phase I trials. These are carboplatin (29) and ado2elesin (37) (35). Carboplatin s approval was based on its comparable efficacy to cis-platinum (28) and its more favorable toxicity profile, ie, reduced and delayed episodes of emesis, reduced ototoxicity, etc. On the other hand, ado2elesin, a totally synthetic analogue of natural product CC1065, has demonstrated a similar potency and antitumor activity profile as its natural prototype but is devoid of the delayed death UabiUty associated with the parent dmg in animals (36). 

Passive transdermal dehvery systems on the market tend to be either matrix or membrane controUed. In matrix devices, the stmctural and molecular characteristics of the dmg-polymer matrix determine dmg release. Examples of polymer matrix-controUed diffusional systems for angina prophylaxis include Nitro-Dur and Nitrodisc, which provide transdermal dehvery of nitroglycerin [55-63-0], and Erandol, a tape that releases isosorbide dinitrate [87-33-2]. Matrix diffusional systems have been used for dehvering dmgs with a wide therapeutic index. 

The antimalarial properties attributed to preparations from Dichroa febrifuga by the Chinese were confirmed about 1944 and two alkaloids, febrifugine (999) and isofebrifugine (1000), were isolated eventually. After difficult structural elucidations, syntheses of ( )-febrifugine followed it proved to be half as active as the natural material, itself far better than quinine, but the therapeutic index was disappointingly low (67HC(24-l)490). 

Uracil is used more effectively, in nucleic acid synthesis within a rat hepatoma than in normal liver. This observation appears to have stimulated the synthesis of 5-fluorouracil (1027) as an antimetabolite mainly because the introduction of a fluorine atom involves a minimal increase in size. In the event, 5-fluorouracil did prove to have antineoplastic activity and it is now a valuable drug for treatment of tumors of the breast, colon or rectum, and to a lesser extent, gastric, hepatic, pancreatic, uterine, ovarian and bladder carcinomas. As with other drugs which interfere with DNA synthesis, the therapeutic index is quite low and great care is required during treatment (69MI21301). 

In the treatment of human neoplastic diseases methotrexate has largely supplanted aminopterin in chemotherapy, due to the better therapeutic index of the former in experimental animals, although this superiority over (325) has not been conclusively demonstrated in man. 

The single dose of a drug is mo.stly derived from experience it is only possible in a very few cases to calculate it from the activity of the constituents. However, as many herbal drugs arc only weakly active and contain non-toxic substances, i.e, the therapeutic index is large, exceeding the dose is usually only of minor significance nevertheless, the pharmacist must know what the exceptions are in this book, the sections on Side effects and Making the tea draw particular attention to such cases, c.g. arnica flowers, liquorice root, etc. 

Therapeutic index. Ratio between the median lethal dose CLDso) and the median effective dose CED30) of a drug. 

The applicant should provide justification for using the racemate. Where the interconversion of the enantiomers in vivo is more rapid than the distribution and elimination rates, then use of the racemate is justified. In cases where there is no such interconversion or it is slow, then differential pharmacological effects and fate of the enantiomers may be apparent. Use of the racemate may also be justified if any toxicity is associated with the pharmacological action and the therapeutic index is the same for both isomers. For preclinical assessment, pharmacodynamic, pharmacokinetic (using enantiospecific analytical methods) and appropriate toxicological studies of the individual enantiomers and the racemate will be needed. Clinical studies on human pharmacodynamics and tolerance, human pharmacokinetics and pharma-cotherapeutics will be required for the racemate and for the enantiomers as appropriate. 

CR(C)1419 83GEP3242610] they had a higher therapeutic index as a hypnotic than ketamin. 

The development of ligands selective for individual receptor subtypes relevant to a targeted disease could decrease these toxic effects thereby improving the therapeutic index. Two new arotinoids are already available for topical use in skin diseases. These are tazarotenic acid (tazarotene) and 6-[3-(l-adamanty 1)-... 

Encapsulation of cDDP in liposomes did not show such favorable effects. Liposome encapsulation of cDDP decreased the antitumor effect (Fig. 9). It was demonstrated that administration of cDDP liposomes resulted in a lower incidence as well as reduced severity of focal alterations of the epithelium of the proximal tubuli compared to administration of the free drug (Steerenberg et al., 1988). However, despite this reduction in renal toxicity the therapeutic index... 

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