102 1 03 1 04 1 05 Molecular weight

Figure 9.50 Correlation between the percentage dose absorbed after nasal administration and molecular weight: 4-oxo-4H-1-benzopyran-2-carboxylic acid (chromocarb) (•); p-aminohippuric acid (A); sodium cromoglicate (O); inulin (■); dextran (♦); r # -0.996.

Reproduced from A. N. Fisher et ai, J. Pharm. Pharmacol., 39, 357 (1987).

effective barrier to the entry of relatively large particles. The division of the nasal cavity exposes the air to maximal surface area. As in the other parts of the respiratory tree, sudden changes in the direction of airflow cause impingement of large particles through iner-tial forces. The respiratory portion of the nasal passage is covered by a mucous membrane which has a mucous blanket secreted in part by the goblet cells. The ciliary streaming here is directed posteriorly so that the nasal mucus is transported towards the pharynx. Figure 9.51 shows the fractional deposition of inhaled particles in the nasal chamber as a function of their particle size. A diameter of not less than 10 ^m minimizes the loss of drug to the lung. In the external nares, removal of particles occurs on nasal hairs; further up iner-tial deposition takes place, and in the more tortuous upper passages deposition is assumed to be by inertia and sedimentation.

Comparison of the nasal route with other routes has been made in some instances. Desmopressin (1-desamino-8-D-arginine vasopressin) administered as a 20 ^g dose elicits a response equivalent to approximately 2 ^g administered by i.v. injection. Also, a greater dose of virus is required to obtain an equivalent response to a nasal vaccine than when administered by other routes.

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