(Refiles to add word "latest" to paragraph 14 to specify trial) (Adds researcher comments, closing share prices)

By Ransdell Pierson

NEW YORK, Sept 18 (Reuters) - Johnson & Johnson <JNJ.N> said on Tuesday its drug Prezista was at least as effective in a late-stage trial as Abbott Laboratories Inc's <ABT.N> Kaletra in cutting HIV to undetectable levels among patients not previously treated with HIV drugs.

"Doctors want to start their HIV patients with the best shot they can, and data from this trial show Prezista is a very attractive drug," with some advantages over Kaletra, said Dr. Karam Mounzer of Presbyterian Medical Center in Philadelphia, who helped lead the trial.

Prezista, a once-daily protease inhibitor recently introduced by J&J, was given to patients along with Abbott's older protease inhibitor Norvir (ritonavir) and Gilead Science Inc's <GILD.O> widely used Truvada. Truvada itself contains two drugs belonging to a separate class of HIV treatments called nucleoside reverse transcriptase inhibitors.

Another group of patients was given Kaletra, a top-selling HIV pill that combines Norvir with protease inhibitor lopinavir, as well as Truvada.

J&J said HIV levels dropped to undetectable levels after 48 weeks among 84 percent of patients taking experimental 800-milligram doses of Prezista, compared with undetectable virus levels in 78 percent of those receiving Kaletra.

Data from the so-called ARTEMIS trial -- the first study to test Prezista among patients never previously treated with other HIV medicines -- were presented in Chicago at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC).

Prezista was launched last year as a treatment for adults whose HIV infections have not been adequately controlled by prior treatment with other HIV drugs. Data from the new trial could eventually support its use as a first-line treatment.

Kaletra -- approved for adults and children over 6 months of age, both as a first-line treatment and for those who have failed to benefit from other drugs -- is the world's best-selling HIV protease inhibitor product. It had global sales last year of $1.14 billion.

"There clearly was a trend in the trial favoring Prezista over Kaletra in terms of viral suppression," Mounzer said, although the difference was not statistically significant.

For a subset of patients with high levels of HIV at the beginning of the trial, Prezista achieved statistical superiority, he said.

The percentage of patients experiencing nausea -- a common side effect of protease inhibitors -- was significantly lower among the Prezista group, Mounzer said.

Another advantage of Prezista, he said, is that it is taken only once a day, whereas a high percentage of patients must take Kaletra twice daily to maximize viral suppression.

Mounzer said Prezista also proved at least as effective as Kaletra in another trial described earlier this year, in which the two medicines were tested among patients who had previously taken other HIV drugs.

Abbott spokeswoman Ilke Arici said Prezista's showing of "non-inferiority" to Kaletra in the latest trial was somewhat questionable because the trial design did not reflect how the drugs are given in real-world medical practice.

For instance, Arici said many patients were given a soft gel form of Kaletra in the study, which she said is less effective than a new tablet formulation now widely used in the United States.

"So this trial does not reflect a real-life scenario," Arici said.

J&J spokeswoman Pam Van Houten acknowledged most patients taking Kaletra began on the soft gel form and were later switched to tablets after that new Abbott formulation became available.

Van Houten noted the label for Prezista recommends patients take two of the 600-milligram tablets, for a total dose of 1,200 milligrams.

But she said a lower total daily Prezista dose of 800 milligrams was used in the new study, on the theory that a lower dose would suffice for such patients who have not previously taken HIV drugs and therefore have not developed resistance to them.

Van Houten said J&J aims by the end of 2007 to ask U.S. regulators, based on the new data, to approve the use of Prezista for patients never previously treated for their infections with the virus that causes AIDS.

Shares of J&J closed up 1.4 percent to $63.83, while Abbott rose 3 percent to $53.60 on the New York Stock Exchange amid strong stock market gains.

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