This section was initially intended to be a review of the EFV resistance reports in Chicago which characterize the resistance profile of EFV. But, I tried to gather a little additional information on DLV and NVP resistance to expand the discussion.

NNRTI cross-resistance is not yet well understood, and additional research is necessary to better understand it. This Resistance section discusses mutations and cross-resistance in more detail than many of you may be interested. If you don’t want to read the entire section, here is a brief summary. The 3 NNRTIs, DLV, NVP and EFV, have resistance mutations in common. The K103N mutation appears commonly associated with resistance to any of the 3 NNRTIs. Appearing less frequently than a K103N mutation, some virus mutants without the K103N appear, which EFV may be able to suppress. A single mutation of Y181C can emerge from NVP or DLV resistance, or a P236L single mutation may emerge from DLV treatment, but EFV was still active against these single mutations in vitro (see below). Conversely, the K103N mutation is highly associated with EFV resistance in vivo, but both NVP and DLV resistance is associated with K103N.

At this point in our knowledge about NNRTI resistance, for the purpose of making treatment decisions now, it may be safe to assume there is at least partial cross-resistance between EFV, DLV and NVP. Treatment decisions should be made prudently.

Resistance to a NNRTI develops more quickly than resistance to a protease inhibitor. A single mutation can cause high level resistance to NVP or DLV. DuPont Merck says high level resistance to EFV may develop more slowly, but high level resistance may not be necessary to cause treatment failure. At this time, it appears as if there’s too little information to know if and how this EFV information will bear out in treatment for individuals.

DuPont Merck reported two studies at Chicago. The in vivo study looked at the circulating plasma virus in patients participating in EFV trials whose viral load rebounded after an initial decline. The most common resistance mutation was the K103N. It is key to the development of EFV resistance. It was observed in 86% to 93% of patients failing EFV combination therapy with either indinavir or AZT/3TC. As well, both delavirdine and nevirapine show loss of activity when the K103N mutation is present. Based on reported in vitro data nevirapine can show a high level loss of activity when this mutation is present. One in vitro study suggested that delavirdine had 7 fold loss of activity when the K103N was present and another study suggested delavirdine developed a high level loss of activity when the K103N was present. Investigators on this DuPont Merck in vivo study said that all the mutations they observed had been seen previously as associated with one or more NNRTIs. Other mutations observed but less frequently than the K103N included Y188L, and G190S. Double mutations observed to occur less frequently than the K103N but more frequently than other double mutants included K103N/V108I, L100I/K103N, K103N/P225H (occurred more frequently) or K103N/G190S. Viruses with double mutations were seen in up to 70% of EFV failures. But it appears as if the K103N is the first mutation to develop.

DuPont Merck also reported results from an in vitro study of recombinant viruses carrying defined mutations. A recombinant virus is one that is artificially created or constructed in the lab to contain certain mutation(s). Resistance by a drug to these recombinant viruses does not necessarily predict responses in humans. However, it appears that some of the in vitro resistance studies for protease inhibitors predicted the cross-resistance we’re experiencing in humans. The in vitro data reported by DuPont Merck (see table below) showed loss of activity (resistance) by efavirenz when the following single and multiple mutations were present in a recombinant virus: S48T/G190S, K103N, L100I, K101E, Y188L, S48T/K103N/G190S. A recombinant virus showed that the K103N mutation was associated with18 fold resistance to EFV. The results of this in vitro study showed that delavirdine and nevirapine also showed reduced activity against almost all of these same mutations (see table below). EFV maintained effectiveness against the single mutations: V106A, Y181C, Y188C, G190A, P236L.

The Y181C mutation can cause NVP resistance, and the P236L mutation can cause DLV resistance. You will note that neither of these two mutations emerged associated with EFV resistance in vivo. Based on in vitro data, it is reasonable to expect that a Y181C resistant virus will remain sensitive to EFV, and it is reasonable to expect a P236L resistant virus will remain sensitive to EFV. But if a double mutant occurs, such as K103N+P236L (which can emerge from DLV) or a Y181C+K103N (which can emerge from NVP), cross resistance may result.

DuPont Merck officials have said they increased the dose of EFV from 200mg once a day to 600mg once daily at least in part because they hoped the increased drug blood levels might suppress virus containing the K103N mutation and other mutations. A major concern in preventing cross-resistance to EFV after prior resistance to DLV or NVP is to be able to suppress the K103N. Individuals may have the K103N mutation from prior NNRTI experience or as pre-existing mutations prior to any NNRTI therapy. DuPont Merck says the K103N has been suppressed for some individuals. The EFV blood levels some individuals achieve may be adequate to suppress the K103N. But, we have not yet seen that for most individuals the 600 mg qd dose can suppress resistant viruses containing the K103N.

A study of in vitro passage of virus in presence of DLV showed the key mutation due to delavirdine in vitro was P236L. But this mutation may emerge only in a small percentage of individuals with DLV resistance. Additionally, a double mutant of K103N+P236L can emerge. In vivo analysis of patient isolates from clinical trials with DLV monotherapy and combination therapy have shown the K103N mutation to be key. Other mutations identified from in vitro analysis of patient isolates from clinical trials include K103T, Y181C, P236L, K103N+Y181C and K103N+P236L. Upjohn says subjects with the K103N and/or the Y181C mutations are likely to be resistant to DLV and NVP. They reported that combination therapy of DLV+AZT was observed to cause resensitization to AZT. In Chicago, a report by Lisa Demeter and the AACTG 261 team suggests that concomitant therapy with DLV and NRTIs may alter the resistance mutation profile that emerges. For example, DLV+ddI may cause the Y181C to emerge while DLV+AZT prevents the Y181C, but the K103N may emerge. Based on the in vivo resistance data reported for EFV, the K103N appears to be the only mutation in common between EFV and DLV.

Genotypic mutations observed in clinical studies with NVP include L100I, K103N, V106A, V108I, Y181C, Y188C/L, and G190A. The Y181C mutation may be the most common one. In vitro studies have shown high level nevirapine resistance when the Y181C or the K103N mutation was present. Multiple mutants observed in an early clinical study include: Y181C+K103N, Y181C+K103N+V108I, Y181C+K101E+G190A. In this study (1), resistance data on 6/7 individuals showed each of the 6 had a multiple mutation, Y181C was present in 6/7 mutants, and a K103N mutation was present in 2/6. Based on both the in vivo and in vitro EFV resistance data reported, NVP and EFV have several mutations in common: K103N, G190A, L100I, Y188L, K101E. Based on the EFV in vitro study data EFV appears to suppress all but the L100I, the Y188L multiple mutation, and the K103N. The same study reported a maximum decrease of 2 log (range 1.96 to 2.43) after 2 weeks in the monotherapy patients, using the Roche Amplicor test.

The EFV in vitro study reported at Chicago and discussed above indicated EFV may suppress virus with a mutation at Y181C or P236L; Y181C is associated with NVP resistance, and P236L is associated with DLV resistance. But as stated above P236L may be present in a small percentage on individuals with DLV resistance and they may have accompanying additional mutations. The same may be said about Y181C and NVP resistance. This brings me full circle to my original statement at the beginning of this section--at this point in research, it is fair to expect at least partial cross-resistance between NVP, DLV and EFV.

Retrovirus abstract #702. The number in parenthesis is the fold resistance. The number outside parenthesis represents the amount of drug (as measured by nM) required to suppress the mutation. Resistance in this table is characterized as the fold increase in IC90 relative to that of the wild type virus for each drug tested. For example, 38 nM of DLV is required for 90% inhibition of wild type virus (NL4-3), and 1000 nM is required to suppress the K103N mutation. That is a 28 fold increase in resistance.

The authors concluded the resistance profile partially overlaps with that of the other NNRTIs tested, although potency is maintained against a number of NNRTI resistant mutants. But, a problem may occur when the K103N emerges as part of a multiple mutant containing one of these other single mutations.

A mutant of Y188L including additional mutations emerged in 4.5% to 6.7% of the sequenced patients in two clinical studies (n=51). A mutant of G190S plus other mutations emerged in 0% to 13.3%. These two multiple mutations occurred relatively infrequently. While a multiple mutant containing a K103N mutation occurred 53.3% to 68.2%. While, as stated above, overall 86% to 93% of patients in the study acquired a K103N mutation.