This article contains a comprehensive report of the latest information on 1592U89 resistance and cross-resistance. The results of two studies reported at the Retrovirus Conference suggest that phenotypic resistance prior to taking 1592 may predict an individual’s response to 1592. The study results showed a correlation between the level of phenotypic resistance to 1592 and viral load response to treatment with 1592, for the individuals in this study. These findings are reported below following an explanation of phenotypic and genotypic resistance testing. In both studies, baseline phenotypic resistance was determined by a company named Virco located in Belgium. Virco uses their phenotypic resistance test called Antivirogram. The study results suggest that if you are treatment experienced, you may want to consider phenotypic testing prior to using 1592.

What is Phenotypic Resistance Testing? A sample of your blood is placed in a test tube along with a specific drug. The lab determines how much drug is needed to inhibit HIV by either 50% or 90%. When you are talking about the amount of drug required to do this you are talking about the inhibitory concentration (IC), so you’ll hear terms like IC50 or IC90. IC50 means the drug can inhibit 50% of HIV in the test tube and IC90 means 90% of HIV is inhibited. This is called phenotypic resistance testing. If you need more drug to suppress the virus than is normally needed that means there is phenotypic resistance. If the amount of drug necessary to inhibit virus replication is 8 times more, than you have 8 fold phenotypic resistance.

What is Genotypic Resistance Testing? A different way to look at resistance is to perform a genotypic analysis, where you are looking at the sequence of the genes in the virus itself. You are looking for changes in the genes called mutations; but, you have to look for mutations that have been proven to result in drug resistance, to make the findings useful for the patient.

Some researchers have expressed doubts about whether or not genotypic and phenotypic test results can provide enough information for making reliable treatment decisions at this time in the development of the tests. The longer a period of time you’ve been off a drug the less likely you are to detect resistance. For example, if you stopped indinavir 6 months prior to taking a phenotypic test you may not detect indinavir resistance, but it might emerge once you start a different protease inhibitor. There may always be resistance below the surface that you may not detect. Of course, it may not always be relevant. Unfortunately, we are unsure how much fold resistance is associated with loss of antiviral activity associated with each drug. We don’t know what the breakpoint may be. For example, a phenotypic test result may show 5 fold resistance to a drug but we may not be sure how much loss of antiviral activity is associated with 5 fold resistance. Five-fold resistance for a particular drug could mean partial loss of activity or full loss of activity. In some cases genotypic testing will give you more useful information than a phenotypic test will. If you have 5 AZT mutations genotypic testing can tell you all of them. Using both tests gives you the most information, but is expensive.

We are in the early stages of the development of these technologies, as we were with viral load testing several years ago. Studies are needed to establish the correlation between genotypic testing results and phenotypic testing results and their predictability of failure or success for a specific drug. These studies have started and initial results expect to be presented in Geneva. NATAP will report the information.

Up until now phenotypic testing was only available by shipping your blood sample to Virco in Belgium for analysis. On April 28, it was announced that LabCorp, a commercial national laboratory, would have exclusive rights in the US to offer the Virco phenotypic test as well as Virco’s newly available genotypic test to doctors and patients. The test will be available starting in July ‘98. At this moment, the Virco phenotypic resistance test costs about $800 but the price policy is being reviewed. Virco is installing a new system for processing blood samples. If the new system is properly in place, Virco believes they will be able to reduce the current turnaround time of 3 weeks to 14 days.

Several other companies, such as a San Francisco based company called Virologics, are developing phenotypic resistance tests but they are further behind in development. Virologics may be making their testing commercially available by the end of this summer.

Initially, the blood samples collected by LabCorp will be shipped back to Belgium for processing. But the expectation is that sometime this coming summer arrangements will be made to perform some of the processing steps in the USA. If demand is high enough arrangements may be made to complete all the processing in the USA.

Aside from the two 1592 studies reported in this report, Virco is conducting and planning a number of studies to try and define the role that phenotypic resistance testing can play in treatment decision making. They will examine if there is a correlation between phenotypic resistance testing results, the prediction of sensitivity to certain drugs, and its predictability for clinical outcome. These studies include looking at phenotypic resistance during primary or acute infection as well examining its role in making salvage therapy decisions. After treatment failure, using a phenotype test it may be possible to identify which drugs a person has become resistant to and which drugs they may remain sensitive to. The testing may also be able to predict which drugs the person should switch to. Being able to use the testing in these ways depend upon the results of the validation studies in predicting drug sensitivity and clinical outcome. Again, at the moment some doctors and researchers are skeptical of the reliability in using these tests to make treatment decisions.

Virco is working with the ACTG and other organizations in conducting both prospective and retrospective studies. Retrospective studies, where researchers look back at stored blood samples, will attempt to correlate clinical endpoints (AIDS events and death) with phenotypic resistance. These studies include a look at the pharmaco-economic aspects of using phenotypic testing to show that using the technology will be a cost saving to insurers, so that monies won’t be wasted on therapies that are not effective. Future cost will in part depend upon the usefulness of the testing results and demand for them. Some initial results from the studies outlined above will be reported at the International AIDS Conference in Geneva.

Kaiser Permanente is the largest HMO in the USA with over 9 million subscribers They are collaborating with Virco in the clinical study effort to establish the test’s utility. In a study of individuals relatively heavily pre-treated with antiretroviral therapy, participants will be randomized to new combination therapies. One arm will make switches based on using Virco’s phenotypic resistance testing. The other arm will be blinded to the phenotypic resistance test results and will make decisions on treatment switching based on usual considerations. The study goal is to look at both clinical outcomes (AIDS defining events) and virological outcomes (viral load changes) in assessing the effect of using the test results. In addition, Kaiser will be examining the economic aspects of using this expensive test. They will explore if it saves money over the long run to use this test in identifying appropriate treatments for a particular individual or for specific populations in general.

Dr David Melnick, of Kaiser, says that in their experience, the cost of combination therapy has increased 5 fold but the cost for patient care (number of hospitalizations, and length of stay in hospital) has declined, creating an offset whereby overall costs increase at a rate of about 3% per year. This is in large part due to a cut by 50% of individuals with <50 CD4. Patient care costs are highest for individuals with <50 CD4. In an abstract presented by Dr Melnick at the Chicago Retrovirus Conference, he reported an analysis of patient costs per year based on CD4 count: $3930 (>500 CD4), $9238 (200-500 CD4), $19,541 (<50 CD4). Although Virco’s phenotypic test may be costly, it may prove to be cost effective to insurers.

1592U89 Resistance and Phenotypic Resistance Testing. The authors of the studies reported at the Retrovirus Conference concluded "high level resistance to 1592 (>8 fold) and/or multiple genotypic mutations known to confer NRTI phenotypic resistance are associated with poor viral load response to 1592. Phenotypic testing seems to be a simple tool for predicting viral load response to 1592, but further clinical trials are required to confirm these findings." For the most part, the authors say the number of NRTI genotypic mutations known to be associated with NRTI resistance also predicts the response to 1592, although there was one possible exception (pt# 498, see tables below). Higher levels of dual AZT/3TC phenotypic resistance and baseline phenotypic resistance to other NRTIs also appear to be significant contributing factors to a poor response to 1592. In other words the degree of response to 1592 may be less if you have extensive NRTI resistance which includes dual AZT/3TC resistance.

A more detailed discussion about 1592 resistance based on previous in vitro research is available in the NATAP Reports Jan ’98 issue. Briefly, resistance to 1592 in vitro is associated with the emergence of the 184 mutation followed by mutations at 65, 74 and 115. Each of these single mutations resulted in no more than a 4-fold increase in the IC50 (phenotypic resistance). 8 fold or higher resistance occurred only with the emergence of 2 different triple mutations (65, 74, 184 or 74, 115, 184) or with 1 double mutation (74,184).

In the first study presented by Dr. Randall Lanier and others at Glaxo Wellcome, they analyzed the baseline and week 24 data for participants in study CNAA2003. These were individuals with prior nucleoside experience (see our January newsletter, NATAP web site for more details about the study) who were taking only nucleoside therapy. For 2003 they continued on their current nucleoside therapy and added only 1592 300mg bid. In addition to baseline phenotypic resistance, study participants were assessed for their baseline genotypic resistance using the sequencing kit from Applied Biosystems, and the amount of prior nucleoside experience they had.

At week 24, 9/15 individuals (60%), had >1 log reductions in viral load following the addition of 1592 to their current NRTI therapy. 3/15 had viral load reductions >3 log. These 3 individuals had virtually no 1592 baseline phenotypic resistance. They had prior nucleoside experience but <4 fold baseline phenotypic resistance to other NRTIs (d4T, ddI, 3TC, AZT, ddC). 10/15 (67%) individuals had >0.5 log reduction in viral load after adding 1592 to their current NRTI therapy; 5/15 had <0.5 log reductions.

The study participants were divided into 3 groups by order of their baseline 1592 phenotypic resistance: <4 fold, 4 fold to 7 fold, and ³ 8 fold resistance.

Table 18. <4 fold 1592 Phenotypic Resistance at Baseline

Column 1 is the NRTI treatment the person was receiving just prior to the addition of 1592, and the number of months of experience with the treatment. Columns 2-7 give the fold baseline phenotypic resistance to that particular NRTI. For example, in the first row pt# 445 has 1 fold resistance to each of the NRTIs listed. Wherever na is placed, that means the information was not available. The last two columns refer to the median viral load changes at weeks 4 and 24. The week 24 columns in all 3 tables also contain the baseline genotypic mutations detected for that individual.

In the following tables you will see genotypic mutations. It is helpful to know for which NRTI(s) these mutations can contribute to resistance:

In Table 18, only one person (pt# 497) had dual AZT/3TC resistance. All the others had <4 fold 1592 baseline phenotypic resistance and resistance to at the most only 1 other NRTI. Higher levels of dual AZT/3TC resistance is associated with less likelihood that a person will respond well to 1592. However, having used AZT previously for a longer period of time does not necessarily predict higher level AZT resistance.

Table 19. 4 fold to 7 fold 1592 Baseline Phenotypic Resistance

As you can see individuals with higher baseline phenotypic 1592 resistance (5-7 fold) have more phenotypic resistance to AZT, 3TC and/or other NRTIs and in this study these individuals don’t respond as well as those with <4 fold baseline resistance. Several individuals have enough d4T resistance that you might expect to see a d4T related mutation; although the 75 mutation has been observed to be associated with d4T use, usually no resistance mutation has been observed to be associated with d4T failure. You can see in this table that every person has at least 2 NRTI mutations, while in Table 1 only 4/14 individuals had genotypic mutations.

Table 20. ³8 Fold 1592 Phenotypic Resistance at Baseline

3/4 individuals had no VL decrease in response to 1592. The other person had a small response (-0.36). All 4 of these individuals had high level dual AZT/3TC resistance, accompanied with some resistance to other NRTIs. As you can see there is a trend towards decreased response to 1592 at higher levels of baseline phenotypic resistance to 1592.

Table 21. Overall Week 24 % Viral Load Responses

The factors that appear to effect response to 1592 are:

• The level of baseline phenotypic 1592 resistance (<4 fold, 4-8

fold, or >8 fold)

• NRTI genotypic mutations

• Higher levels of dual AZT/3TC resistance appear associated

with reduced response to 1592

• Baseline phenotypic resistance to other NRTIs appears

associated with response to1592

In Chicago, Glaxo Wellcome held a community meeting separate from the Conference where they told us that of the first 200 participants in the 1592 expanded access program, only 25% had >0.5 log drop; and, only 40% of these 25% (10% overall) had >1.0 log drop at month 2. The individuals who did best added a NRTI which they had not taken in last year. 1,500 individuals have enrolled in the program and they said these first 200 were representative of the remaining individuals. It is important to put these results in perspective. The criteria for entry into the program was a CD4 count <50, so these were individuals with advanced HIV and few unexhausted treatment options. They probably had extensively used all NRTIs and failed protease inhibitor therapy. It is possible these individuals would have fallen into the group in Table 20 (>8 fold 1592 baseline resistance). If you are treatment experienced, you should be cautious in deciding how to use 1592, so that you can maximize its benefit to you. Phenotypic testing may help you identify your response to 1592. Additional resistance information will be forthcoming in the near future.

A second study using the Virco phenotypic assay technology called Antivirogram was reported by Dr John Mellors and others. The assay was performed on 943 patient blood samples obtained from clinical trials (eg, CEASAR) or from individual patient monitoring. Phenotypic resistance was determined for the following NRTIs— 1592, AZT, 3TC, ddI, ddC, d4T.

As in the previous study, patients were grouped by 3 categories of resistance: <4 fold, 4 to 7, and >7 fold.

The authors concluded the Antivirogram can identify patients who are most likely to benefit from 1592. The full data set from the study reported in Chicago are available on the NATAP web site.

• If a person is found to be sensitive to 1592 by using the Virco Antivirogram phenotypic resistance test they are likely to benefit from 1592 treatment. The most sensitivity is defined as having <4 fold baseline phenotypic resistance to 1592 (median VL reduction in first study was -2.38 log). Intermediate sensitivity is defined as 4 to 8 fold 1592 resistance (median VL reduction was -1.47 log). If a person has >8 fold 1592 resistance, that is predictive of minimal responses to 1592.

• It is not known whether there is a clinically significant difference

between isolates with <4 fold resistance and isolates with 4 to 8 fold resistance.

• For those isolates with AZT/3TC resistance (>10 fold) and no

resistance to additional NRTIs, 71% have <4 fold resistance to

1592.

• Over 95% of those isolates with >10 fold resistance to AZT or

3TC alone remain sensitive to 1592 (<4 fold resistance).

• If resistance to AZT, 3TC or AZT+3TC is associated with other NRTI resistance, sensitivity to 1592 decreases. The sensitivity declines as there are increasing numbers of NRTIs that the isolate is resistant to. None of the isolates with dual AZT/3TC resistance and resistance to 3 additional NRTIs were <4 fold resistant to 1592, but 29% (n=7) had intermediate (4-8 fold resistance to 1592); and 71% of these isolates had ³8 fold 1592 resistance. However, "it remains to be determined whether reduced 1592 sensitivity results mainly from exposure to AZT/3TC or whether it is a more general phenomenon resulting from regimens of NRTIs."

Dr Schlomo Staszewski reported preliminary data from this dose ranging trial of 60 persons randomized to 1592 at doses of 100 mg, 300 mg, or 600 mg bid. Participants who either completed 24 weeks of 1592 or met switch criteria based upon viral load, CD4 or new AIDS defining events had the option to switch to open label 1592 (300 mg bid) plus AZT/3TC. Glaxo Wellcome is conducting a separate study comparing a triple NRTI regimen containing 1592 versus indinavir+2 NRTIs.

55 persons entered the open label phase; 46/55 remained on 1592+AZT/3TC; 1 stayed on 1592 alone; 3 added protease inhibitors to 1592/AZT/3TC; 1 person substituted d4T for AZT; 4 persons were lost to follow-up.

The median reduction in viral load (n=17) at week 28 from the time individuals switched to open label 1592 where they added therapy was between 2 to 2.5 log. Estimations are made based on visual observation of line graph. Ignoring the treatment switches and their timing, at week 48 about 62% (n=46) had viral load <400 copies/ml, and about 42% (n=46) had viral load <50 copies/ml. At week 32, the median increase in CD4 (n=22) was about 150 cells.

Adverse Events. Of the 55 individuals in the study the number of experiences of the most frequently seen side effects were reported: nausea/vomiting (19), malaise/fatigue (11), headache (9), diarrhea (9), sleep disorders (6), GI discomfort/pain (7), dizziness (6).

In a separate study, 72 of 79 participants enrolled into an early dose escalation study of 1592 were required to interrupt therapy following 12 weeks due to limited animal toxicology data. Six persons received 1592 uninterrupted. 43 of the 72 elected to restart 1592 (300 mg every 12 hours) following interruption of up to 1 year. They were permitted to restart 1592 in combination with any additional antiretroviral therapy. When extended therapy started 16/42 had viral loads <400 copies/ml. At week 12, 27/38 had <400 copies/ml. The number of persons undetectable (<400 copies/ml) at week 24 for NRTI combinations only with 1592 were 14/18, and 9/10 for those taking 1592 with a protease inhibitor.

At week 48, the median reduction in viral load and increase in CD4 was about 2 log and about 100 cells for the group taking 1592 with NRTI(s), and about 2.4 log and 250 cells for the group taking 1592 with a protease inhibitor+NRTI(s). At week 48, 8/15 of individuals taking 1592+NRTI(s) were undetectable (<400 copies/ml). The percent was higher (60%, 17/27) at week 36. At week 48, 9/10 individuals taking a protease+1592+NRTI(s) had <400 copies/ml. Investigators reported that study participants in both groups did not experience increases in LFTs, and did not experience changes in hemoglobin.

Preliminary week 16 data was reported by Dr. John Mellors from a 48 week phase II study in which 78 individuals were randomized to 1592 300 mg bid combined with one of 5 different protease inhibitors: indinavir, ritonavir, saquinavir, nelfinavir and 141W94 (amprenavir). The participants were treatment naive. This was an open label comparison where participants were stratified to below or above 100,000 copies/ml.

Viral Load Changes at week 16. Viral load was evaluated by Roche Amplicor (400 copies/ml limit of detection) and Roche Ultrasenstive assay. Baseline median CD4 and viral load were 349 cells and 4.74 log or about 60,000 copies/ml. The median viral load reduction at 16 weeks for all 4 arms was at 400 copies/ml. Levels below 400 were assigned a value of 400 copies. This is common practice. See Table 22

The differences in the study arms are not statistically significant, and the number of evaluable participants is relatively small particularly in the <50 copies/ml evaluation. It can take longer than 16 weeks for some individuals to reach 50 copies/ml. Mellors reported that at week 24 the proportion of persons reaching <400 copies/ml was increasing. 5 individuals have shown an increase in viral load from nadir (lowest point reached), but none equal to .50 log. Four of them were in the saquinavir arm and one was in the indinavir arm. The total CD4 increases range from 50 to 150, including an increase of 20 naive CD4 and 100 memory CD4 from baseline to week 16. Compliance assessments are ongoing. Phenotypic and genotypic testing are being performed on the rebounders.

Adverse Events. There was one drug reaction with dehydration and diarrhea. The person required hospitalization. There was one extreme grade 4 lab abnormality of hypercholesterolemia. There was case of neutropenia.

There were 4 eruptions that are now known to be characteristic of an acute 1592 hypersenstivity reaction. This reaction is discussed in the following paragraph. Dr Mellors described one patient with systemic symptoms: nausea, vomiting, general malaise and 12 hours after stopping medication developed a skin eruption. In this study the 1592 reaction occurred in 5% (4/78) but overall there is a 2%-5% incidence across 1592 studies. The onset is after 3-42 days (median 9 days). The characteristics are systemic: malaise, nausea, vomiting, fever, with or without rash. It resolves in one to two days after discontinuation of the drug. Dr Mellors said that treating through it is very difficult. Only one patient has been able to treat through it.

A hypersensitivity reaction has been reported due to taking 1592 at an incidence rate of about 3% (reported range 2-5%). If a person has the hypersensitivity reaction to 1592 they are to stop taking the drug, and they cannot take it again. Symptoms resolve rapidly with drug discontinuation. Restarting 1592 after experiencing the hypersenstivity reaction can result in serious complications. As a result of restarting therapy after experiencing hypersensitivity, some individuals have been hospitalized and there is one reported death. It is very important to clearly understand this reaction and to be able to recognize it. It is not dose dependent and is characterized by a fever first accompanied by one or more of the following: nausea (and/or vomiting), malaise (fatigue or tiredness), rash. Additional effects that can be experienced are swollen lymph glands, diarrhea, and muscle aches. If you think you may be experiencing this reaction please consult with your doctor immediately.

CSF. Dr Josh Ravitz, of Glaxo Wellcome, reported data from animal studies of CSF penetration by 1592. Results from in vitro (laboratory) and in vivo animal studies suggest that 1592 should have brain penetration similar to AZT. The mean concentration of 1592 CSF levels in humans (n=4) after a 200 mg dose of 1592 was about twice the IC50 (the amount of drug necessary to suppress virus replication by 50% with clinical isolates).

Data from studies of several protease-sparing NNRTI (EFV, DLV, NVP) regimens have been reported at the Retrovirus Conference. This open-label pilot study explores treatment with 3 NRTIs during primary infection using AZT, ddI and 3TC. The 9 study participants consented to discontinue therapy after month 12. The investigators reported that at month 12 during therapy, 9/9 patients had <500 copies/ml (Chiron bDNA) and <5 infective cells/107 cells for virus in PBMC cells in blood. The CD4 counts increased significantly, about 200, and plateaued at month 2-3. After discontinuation of therapy, HIV was detectable again in plasma and PBMC in all participants.

The baseline viral load was 461,100 copies/ml, ranging from (28,100 to 2.715 million). The baseline CD4 was 438 (range 238-707). The baseline value for blood cellular virus in PBMCs was 966 infected cells/107 (range 240-3450). The mean follow-up was 18 months (15-33). Plasma viral load became detectable in all patients after a mean time of 3 months (1-9) post discontinuation. There were no side effect caused discontinuations. 1/9 discontinued ddI for personal reasons.

We don’t know how long would the participants have remained undetectable, if they had continued therapy, because all 9 consented to discontinue therapy at month 12.

Other studies are exploring triple NRTI combinations. There is a 300 person study ongoing in treatment naive individuals comparing a different triple NRTI regimen of ddI+d4T+3TC to indinavir+d4T+ddI and nevirapine+d4T+ddI. Preliminary data is expected to be available in Geneva in June. Individuals failing the nevirapine or indinavir arms will be offered a rollover regimen which will include AZT/3TC. So, this study is evaluating AZT/3TC after failing d4T/ddI. This may help answer some of our questions about sequencing NRTIs. As well, the triple NRTI combination of AZT/3TC and abacavir (1592) is being explored in a separate study.

Based on the preliminary 6 month data from these open-label and randomized studies in treatment naive individuals, treatment with any 3 of these double NRTI combinations appears to result in similar viral load reductions and CD4 increases. The study is planned to continue for 48 weeks.

Dr. Kathleen Squires reported the preliminary 24 week data from this study. Median baseline CD4 and viral load was 428 cells and about 46,000 copies/ml for the d4T/3TC/IDV group versus 328 cells and about 27,000 copies/ml for the AZT/3TC/IDV group.

Discontinuations. In the d4T/3TC arm 7/49 (14%) discontinued prior to this analysis. The reasons were: lost to follow-up (5), physician decision (1), and pregnancy (1). In the AZT/3TC arm there were 11/51 (22%) discontinuations prior to this analysis. Reasons were: lost to follow-up (5), physician decision (1), subject voluntarily withdrew (2), adverse event/subject request (1), death (1), subject moved (1).

Viral Load and CD4 Responses. At week 24, the d4T/3TC/IDV arm (n=40) appeared to have a 1.9 log reduction from baseline based on a visual observation of the line graph, and the AZT/3TC/IDV arm appeared to have a 1.6 log reduction (n=39). The Chiron bDNA viral load assay was used. At week 24, 87% (34/39) in the d4T/3TC arm had undetectable viral load (<500 copies/ml) and 80% (32/40) had undetectable viral load in the AZT/3TC arm. At week 24, the median CD4 increases from baseline appeared on the line graph to be about 140 for the AZT/3TC group and about 175 for the d4T/3TC group. 7 participants (7%) never achieved <500 copies/ml (3 on d4T/3TC, 4 on AZT/3TC). 16 participants (16%) had <500 copies/ml but rebounded, with 8 being in each group (2 consecutive measures >500 copies/ml).

Lab Toxicities. ALT (liver enzyme function test)- 3/49 in the D4T/3TC arm vs 1/51 in the AZT/3TC arm had grade 3 or 4 levels of ALT. Triglycerides- 2/100 individuals (both in AZT/3TC arm) had grade 3 or 4 levels.

Dr. Joe Eron reported preliminary 24 week data for this study. Median baseline CD4 and viral load were 454 cells and about 27,000 copies/ml for the d4T/ddI/IDV group, versus 439 cells and about 36,000 copies/ml for the AZT/3TC group.

Viral Load and CD4 Responses. At week 24, it appeared on the line graph that a 1.65 log reduction was achieved for both groups (n=34 for d4T/ddI arm and n=30 for the AZT/3TC arm). Using the bDNA Chiron viral load test with a limit of detection of 500 copies/ml, at week 24, 68% (23/34) were undetectable in the d4T/ddI arm, versus (77%) 23/30 in the AZT/3TC arm. At week 18 it was 78% (d4T/ddI) vs 66% (AZT/3TC), so the effect on viral load may not yet be distinguishable.

At week 24, the median CD4 increases from baseline were about 140 for the AZT/3TC (n=35) group and about 210 for the d4T/ddI group (n=38). 8% (n=8) never reached undetectable (2 in the d4T/ddI group; 6 in the AZT/3TC group). 18% (n=18) achieved <500 copies/ml but rebounded (8 in the d4T/ddI group; 10 in the AZT/3TC group).

Lab Toxicities. For ALT (liver enzyme function test), 5/50 in the d4T/ddI were reported to have a grade 3 or 4 ALT level, versus 1/50 in the AZT/3TC arm. Both arms had 2 individuals with grade 3 or 4 triglyceride levels. 3/50 in the AZT/3TC arm and 1/50 in the d4T/ddI arm had grade 3 or 4 serum glucose levels. 2/50 in the d4T/ddI arm and none in the AZT/3TC arm had grade 3 or 4 levels of GGT (a measure of liver function).

Two of these new NRTIs (FTC and FDDA) are being tested in early human studies while the first human studies for BCH-10652 are being planned to begin soon.

Dr. Franck Roussseau, of Triangle Pharmaceuticals, reported that FTC is a nucleoside analogue resembling 3TC, but in the lab it’s been consistently more potent than 3TC (about 4-10 fold greater in vitro). It is cross-resistant with 3TC. The data below is from a phase I/II study exploring two doses over a 14 day period. 8 HIV-infected individuals were enrolled in each of these two dose groups: 25 mg bid FTC, 200 mg once daily FTC. Additional dose groups are planned for studies. A pharmacokinetics study was conducted as part of this study and investigators found the estimated half-life to be 9 hours. The baseline mean CD4 and viral load for the 25 mg bid dose group (n=5) was 592 cells and 15,500 copies/ml. The mean baseline CD4 and viral for the 200 mg bid dose group (n=5) was 374 cells and 52,500 copies/ml. See Table 23

BioChem Pharma reported early preclinical information about this new NRTI at the Retrovirus meeting. They reported that they found it to be safe in 3 animal types: rat, mice, and monkey. Bioavailability was reported to be 80% in the animals tested. In the CSF in an animal it was found at concentrations higher than 3TC and AZT. Initial human studies are now being planned.

Investigators reported that BCH-10652 may have a preferable resistance profile. BCH 10652 may have activity against 3TC and AZT resistant viruses. Investigators reported that BCH-10652 was sensitive in vitro to virus resistant to 3TC, ddC, PMEA, AZT and combinations of those drugs. In the in vitro experiments the company showed, BCH-10652 was sensitive to half of the 3TC resistant viruses tested, and was not sensitive to the other half of 3TC resistant virus tested. They cautioned more work is needed to characterize the response to 3TC resistant viruses. They reported the drug was sensitive to the viruses tested which were AZT resistant. Investigators are suggesting that resistance to BCH-10652 may develop slowly. After 12 passages in vitro, so far no mutation in the reverse transcriptase gene has been identified and no phenotypic resistance has been observed.

Drs. Lauri Welles and Richard Little and others at the NCI and NIH reported early data from an ongoing phase I human study of FddA. The drug was initially synthesized and the initial preclinical development was by the NCI (National Cancer Institute), but it is now being co-developed by the NCI and US Biosciences Inc. FddA is similar to ddI but it is a capsule, a buffering agent will not be necessary, and so far they haven’t seen any elevated lipase (pancreatitis) which is a potential concern associated with ddI. The investigators reported FddA was well absorbed by the oral route whether given with food or in a fasting state. It was well tolerated in short term therapy at the highest doses reported tested (3.2 mg/kg every 12 hours). They have not yet reached a dose that cannot be tolerated. It showed antiviral activity in patients with substantial prior NRTI experience. Further studies are being planned. The ongoing trial described herein will be extended by recruiting a new group of patients to receive FddA dosing in combination with d4T and nelfinavir. Increased dosing to 4.5 mg/kg and 6.4 mg/kg will probably be explored, as well as once daily dosing.

In preclinical animal studies, FddA was found to be well tolerated at doses that associated with anti-HIV activity, and had good oral bioavailability. In vitro, FddA was active against viruses resistant to NRTI(s) including viruses resistant to multiple NRTIs. Resistance to FddA may be slow to develop. So far after 12 weeks into the clinical study investigators have been unable to detect FddA resistance. In vitro, strains of HIV with Q151M and multidideoxynucleoside resistance remained sensitive to FddA. In the phase I human study, one patient with 151 multi drug resistant virus had a .40 log reduction in viral load. The intracellular half-life appears to be long at 20 hours suggesting dosing of once or twice daily will provide good antiviral activity.

The phase I dose escalating trial enrolled 24 symptomatic HIV infected individuals for an initial 12 week period to assess safety, tolerability, pharmacokinetics and the drug’s effect on CD4 and viral load. The mean CD4 count was 190 (range 4-418); 5 individuals were antiretroviral treatment naive, 1 had <6 months prior experience, and 18 had >6 months prior experience.

Results. At the highest dose fully tested (1.6 mg/kg), a median reduction (n=6) in viral load was reported of -0.44 log (range 0.2 to 1.3 log). At the 1.6 mg/kg dose investigators reported a trend towards increased CD4 counts in the first 12 weeks of the study. The investigators reported the following prior therapies and viral load changes for 13 study participants receiving one of three doses: 0.8 mg/kg, I.6 mg/kg, or 3.2 mg/kg. Two individuals received 3.2 mg/kg, the highest dose used in this study, and achieved -0.79 and -0.42 log reductions in viral load at week 6.

See Table 24

Safety. The investigators reported FddA to be well tolerated in this short-term study. They characterized all of the following toxicities as possibly related to FddA. One person who developed bacterial pneumonia while on FddA experienced grades 1and 2 neuropathy, grade 3 neutropenia, grade 3 elevated LFTs, grade 2 anemia and grade 1 decreased cardiac output. The following toxicities were also reported as possibly related to FddA: neuropathy (1patient, grade 1), elevated amylase (1 patient, grade 1), neutropenia (1 patient, grade 3).

3/4 individuals had no VL decrease in response to 1592. The other person had a small response (-0.36). All 4 of these

individuals had high level dual AZT/3TC resistance, accompanied with some resistance to other NRTIs. As you can see

there is a trend towards decreased response to 1592 at higher levels of baseline phenotypic resistance to 1592.