The first drug that is thought to prevent a deadly pneumonia in AIDS patients will be made available to seriously ill patients next week without formal Federal approval for marketing.
The drug is being used under a Federal program to give the seriously ill access to promising new drugs before they receive marketing approval from the Food and Drug Administration.
The decision will send a signal to doctors that the drug agency is convinced by preliminary evidence that the drug will prove effective.
The drug, aerosol pentamidine, acts by stopping the growth of organisms that cause Pneumocystis carinii pneumonia, a leading killer of AIDS patients. Several drugs, including an injected form of pentamidine, have been available to treat the pneumonia, but none have been proved effective in preventing it.
The drug agency has been strongly criticized by some advocates for AIDS patients for withholding promising drugs until it had strong evidence of their safety and efficacy. The new decision was perhaps the most significant one yet in an evolving policy under which the agency has said that it will make promising but still unproved drugs available to patients with life-threatening illnesses.
''This could be a landmark decision,'' said Dr. Jerome Groopman, an AIDS researcher at New England Deaconess Hospital in Boston. Prospects of Full Approval
The agency's limited approval of the drug is expected to provide incentives for people who are at risk for AIDS virus infections to be tested and to have their immune systems monitored. If their immune systems reach the critical danger point that places them at high risk for Pneumocystis pneumonia, they can use aerosol pentamidine to ward off the pneumonia.
The decision of the agency, to be officially announced next week, was reported yesterday by several newspapers. The agency would not comment on the prospects for full marketing approval of aerosol pentamidine.
Pneumocystis pneumonia ''is the most common serious infection'' to strike AIDS patients, said Dr. Edward M. Bernard of the Memorial Sloan-Kettering Cancer Center in New York. The once rare form of pneumonia strikes more than half of all AIDS patients and has been the most common immediate cause of death among people suffering from acquired immune deficiency syndrome.
Dr. Bernard said most patients survived their first bout with the disease if they went to treatment centers that are experienced in treating it. But treatment involves three weeks of hospitalization, and some patients who have had the disease two or three times are no longer able to tolerate the powerful drugs used to treat it, Dr. Bernard said.
The drug will be released under a program involving investigational new drugs for treatment, or treatment I.N.D.'s. These drugs can be legally prescribed, but their use must be monitored and more data on the drug's safety and effectiveness must be accumulated before the drugs can be licensed for general marketing.
The only drug licensed for use against the AIDS virus itself, azidothymidine, or AZT, was also initially released in late 1986 in an I.N.D. program after initial studies showed it prolonged life. The drug agency says it has since taken steps to further speed availability of promising drugs against serious diseases. It has also ruled that companies may charge patients for drugs distributed under the program. But critics have complained that the agency's policies remained too restrictive. Prevention of Pneumonia
Patients who have already had one episode of Pneumocystis pneumonia are especially likely to get it again, said Dr. Gifford Leoung of San Francisco General Hospital and the University of California at San Francisco. He and his colleagues found that 45 percent of patients who develop the pneumonia will get it again within nine months if they do not take aerosol pentamidine to prevent it.
Preliminary evidence from Dr. Leoung's group and others indicates that aerosol pentamidine can prevent a second attack of the pneumonia in patients who have already had one bout with the disease.
Although these studies did not demonstrate that the drug also prevents a first episode of the pneumonia, the F.D.A. considered data from a Federal study showing that patients with seriously weakened immune systems, those with counts of a type of white blood cell called T-4 cells below 200 per milliliter of blood, are very likely to develop the pneumonia. The agency suspects that those patients too would benefit from the aerosol pentamidine and is making the drug available to them.
Aerosol pentamidine has been widely used over the last year or two by patients in research programs, and by some doctors and patients, concentrated in the East and West coasts, who have acted without Federal approval. $70 a Vial Pentamidine is made by LyphoMed Inc. of Rosemont, Ill, which charges $70 a vial for the drug, enough for one treatment. Some patients have received the drug at no cost in experiments, but doctors charge patients anywhere from $100 to $500 a treatment, said Martin Delaney, a director of Project Inform, a San Francisco-based group that lobbies for speedier approval of new drugs for AIDS patients..