The dosage for any prescribed medicine is the result of compromise.

     The dosage for any prescribed medicine is the result of compromise. All drugs are potentially poisonous, suggesting that the least possible amount should be administered. On the other hand, drugs become diluted in the blood and large amounts are degraded, taken up by healthy tissue, or excreted without reaching the site of disease. Physicians must balance these opposing considerations in determining effective doses.

     To reduce the risks and inefficiency associated with such determinations, ways are now being developed to deliver the needed dose of medicine to diseased tissues while bypassing healthy ones. One promising approach is the loading of medication into liposomes—microscopic sacs (vesicles) made of the same phospholipids that constitute cell membranes.

     A British scientist, Alec D. Bangham, inadvertently produced the first liposomes in 1961 while experimenting with the effect of phospholipids on blood clotting. Because liposomes are made of the same substance as cell membranes, researchers assumed that the liposomes would be nontoxic and would escape recognition and removal by the body’s immune system; the vesicles might therefore have the opportunity to interact with targeted cells in ways that would cause the vesicles to release their cargoes.

     If these assumptions were correct and if liposomes could, as expected, be readily loaded with drugs, all that would be needed to complete this ideal drug-delivery system would be the attachment of tissue-specific molecules to the outside of the drug-laden vesicles. The liposomes, which were also expected to protect their loads from dilution or degradation, would be attached to the target tissues by the tissue-specific molecules.

     Research has shown that liposomes can be loaded with drugs, are nontoxic, protect their loads, and can deliver concentrated doses of medication. Unfortunately, liposomes are for the most part unable to leave the circulatory system and hence are unlikely to reach most cell types. Unencapsulated, or free, drugs typically diffuse through capillary walls into tissues, but liposomes are too large to pass through the walls of most capillaries. In addition, the body’s immune system does recognize the liposomes and attempts to remove them from circulation.

1. The second paragraph serves primarily to

A. explain the difficulties encountered in prescribing effective drug doses
B. introduce liposomes as a possible solution to a problem faced by physicians
C. discuss the reasons for the effectiveness of liposomes in delivering drugs
D. define the term "liposome"
E. summarize early researchers’ findings about liposomes



2. The passage suggests that liposomes could be made more useful as a drug-delivery system if a way were discovered to

A. make liposomes smaller
B. make liposome walls thicker
C. make liposomes of different shapes
D. enable each liposome to carry a greater amount of medication
E. enable liposomes to travel through the entire circulatory system



3. The passage states that there is a risk associated with administering drugs because

A. some patient reactions to drugs are unpredictable
B. some drugs are difficult to administer properly
C. drugs must sometimes be administered in concentrated doses
D. all drugs are potentially poisonous
E. all tissues are at times sensitive to certain drugs



4. According to the passage, researchers hypothesized that liposomes would release their cargoes during interactions with

A. capillary walls
B. blood clots
C. other liposomes
D. targeted cells
E. the body’s immune system