Ch

NH^^^OH CH, anionic derivative of carrier

drug

Figure 4 Design of chlorphentermine-drug conjugates for lung targeting.

Specific uptake by lung tissue is not restricted to lipophilic amines of the type previously mentioned. Certain antibiotics, such as leucomycin A3, show a high deposition in lung tissue at low concentration of drug [113]. A recent report [118] on erythromycin derivatives, in which the 6, 11, 12 and 4'-hydroxyl groups were totally or partially replaced with O-methyl groups, indicated that these compounds, compared to the parent drug, exhibited a marked increased in lung tissue uptake (four to five times greater) after administration into the external jugular vein of rats (Fig. 5). Of note, erythromycin also has a strongly basic nitrogen. It is interesting that such a significant structural change to the erythromycin molecule does not apparently result in a loss of antimicrobial activity. The tissue levels obtained were in the decreasing order: 6,11,12,4'-OCH3 EM > 6,11,4'-OCH3 EM > 6,4'-OCH3 EM=6,11-OCH3 EM=6-OCH2CH3EM > 11-OCH3 EM > EM. The most potent antimicrobial derivative was shown to be 6-OCH3 EM. Some derivatives of steroidal drugs also exhibit better uptake in lung tissue than their parent compounds. Budesonide (Fig. 6) is a glucocorticosteroid that has been used in inhalation therapy for several years [119]. It possesses a 16a, 17a acetal group that makes the molecule less polar and confers on the molecule better uptake properties in lung tissue. (Note:

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