Resistance Associated With Protease Inhibitors and Entry Inhibitors
Resistance Associated With Protease Inhibitors and Entry Inhibitors
The International HIV Drug Resistance Workshop: Basic Principles and Clinical Implications, now in its 12th year, is always an excellent opportunity to see new data regarding resistance to approved and investigational drugs. In this review I will summarize the key studies presented relating to protease and entry inhibitors and focus on their clinical relevance.
Protease Inhibitors
In reviewing the new and investigational protease inhibitors (PIs), let's consider these agents according to their stage of clinical development:
Although many burning questions regarding atazanavir resistance remain, regrettably nothing was presented at the meeting. Key resistance questions are left open regarding this new drug: How commonly is the I50L mutation seen in large series of atazanavir-failing patients, and to what degree are other mutations associated with it? Is this the common mutation also seen in non-subtype B-infected patients? How does the resistance profile of ritonavir-boosted atazanavir differ (if at all) from the unboosted dose? How does boosted atazanavir compare with lopinavir/ritonavir and other boosted PIs in treating PI-experienced patients? The answers to all of these questions will have to be deferred until future meetings.
Tipranavir
Investigators presented a number of studies related to the resistance of tipranavir, an investigational PI now in large phase 3 clinical trials. Data from a study of PI-experienced patients receiving ritonavir-boosted tipranavir were analyzed. Baseline genotypes and drug susceptibility values were correlated with short-term HIV RNA changes to determine predictors of virologic success. The presence of 3 or more mutations at positions 33, 82, 84, or 90 was predictive of a poor response. These were characteristically accompanied by 16 or more additional PI-related mutations. An additional abstract identified L10I, K20M/L/T, M46I, I54V, and L63A/D/T as some of these additional mutations. The authors hypothesize that the additional mutations may be compensatory in nature due to the large impact 33, 82, and 84 have on the enzyme's active site. The term "UPAMs" (universal PI-associated mutations) was coined by the authors for the 4 key tipranavir-related mutations since they found them associated with cross-resistance to many other PIs. There was some controversy at the meeting regarding the appropriateness of the term. In attempting to delineate a clinically relevant cutoff for drug susceptibility, a baseline fold-change of greater than 2 was found to be associated with a poor tipranavir response.
So where do these findings put us with regard to tipranavir resistance? Common, known PI mutations impact tipranavir, and their accumulation results in increasing levels of resistance. Large numbers of mutations are often required to produce very small reductions in tipranavir susceptibility, and a number of key mutations have been identified. Similar mutational patterns often result in far greater reductions in susceptibility to other PIs, although the "clinical cutoff" of 2 for tipranavir is lower than that of some of the other boosted PIs. The absolute number of mutations required to incur a poor tipranavir response is impressive, and the need for multiple key mutations is also reassuring. Still, there is no clear-cut evidence of an advantage over currently approved boosted PIs, and we will have to await the results of the large comparative studies currently under way. Since these studies are being performed in PI-experienced patients with significant resistance, hopefully they will provide considerable insight into the clinical value of tipranavir in this population urgently in need of new drugs.
TMC114
Data on the experimental PI TMC114 were also presented. TMC114 had been studied in a short-term trial in which multiple PI-experienced patients received the drug at various doses, all with ritonavir boosting. After 14 days, TMC114/ritonavir demonstrated a 1.35-log reduction in HIV RNA. Baseline samples showed multiple PI resistance mutations (median 6), with the majority having more than 1 primary mutation. These viruses showed a good deal of reduced susceptibility to currently approved PIs. Their median fold-change in EC50, as compared with wildtype, was 1.8 for TMC114. The investigators were not able to report which of the mutations present had the greatest impact on TMC114 or which mutations characteristically develop after failing the drug. The results of this short, preliminary trial show promise, although much on the resistance characteristics of TMC114 will still need to be determined before we can evaluate how unique a drug it is. In addition, until a safe and well-tolerated dosage is determined and a clinically relevant cutoff defined, comparing TMC114 susceptibility data to data for other PIs provides little insight. A study comparing the crystal structures and thermodynamics of TMC114 to those of other PIssuggested the drug would have activity against multi-PI-resistant viruses. These favorable resistance characteristics would be the result of a tight interaction between the inhibitor and the enzyme and its relatively small volume that is contained within the substrate envelope. Additional groups using similar technologies also presented hypotheses regarding the development of new types of inhibitors with activity against highly mutated protease enzymes. It is hoped that these insights will lead to clinically useful next-generation PIs -- time will tell.
R0033-4649 and P-1946
Preclinical data from 2 additional investigational PI compounds were presented. Follow-up data regarding the previously described R0033-4649 showed activity against viruses containing multiple mutations. In addition, data regarding specific mutational patterns and reductions in susceptibility were shown that suggested good potential activity, if sufficiently high levels can safely be obtained in patients. P-1946, the prototype of a new class of potential PI inhibitors, was also shown to possess favorable resistance characteristics. It was comforting to see many potential new agents in the pipeline.
New PI Mutations
A number of new PI mutations were described, although not all of them appear to have a great impact on resistance. Two additional amino acid changes at the well-known resistance position 84 were described, and it appears that in addition to the known I84V variant, I84A and I84C both are significant mutations. Similarly, I47A joins I47V as a relevant mutation. L23I and K55R were also described but seem to have a small effect on resistance. Although all these new mutations are very rare, knowledge of them will help us further optimize our interpretation of resistance assay results.
Entry Inhibitors
New resistance data on entry inhibitors were limited at the meeting. I will first review new data presented on the recently approved fusion inhibitor enfuvirtide (T-20) and the follow-up compound T-1249, and then briefly mention other investigational entry inhibitors described.
Enfuvirtide (T-20) and T-1249
Laboratory susceptibility studies using different constructs of envelope fragments were performed to determine which areas held the determinants of enfuvirtide resistance. Interestingly, in addition to the HR1 region of gp41, the gp41 HR2 region was also found to influence susceptibility to the drug. A second study suggested some changes in gp120 might also be important. These findings provide additional support for the importance of the HR1 mutations found in previous studies in determining enfuvirtide resistance, but also open the door to continued research into additional areas that appear also to be involved. Further work looking at specific mutations in the gp41 HR2, as well as other sites contributing to fusion inhibitor resistance, will hopefully come soon. In the meantime, we should realize that our understanding of enfuvirtide resistance is not complete and is not yet ready for clinical implementation. Further study and possibly additional diagnostic tools are needed.
Investigators presented resistance data from the previously reported study of T-1249 in enfuvirtide-treated patients. Patients receiving an enfuvirtide-containing regimen, but with HIV RNA levels above 5000 copies/mL, were switched to T-1249 and showed a significant short-term reduction in viral load. Resistance analysis of these viruses at the time of switch showed multiple mutations in gp41 HR1 amino acids 35-45, as might be suspected on the basis of previous studies. In addition, very high levels of reduced susceptibility to enfuvirtide were seen (mean fold-change 150.1), with far more modest reductions to T-1249 (mean fold-change 1.8). A key clinical observation from this trial was the correlation between length of previous treatment on enfuvirtide and subsequent response to T-1249. Patients who had remained on a failing enfuvirtide-containing regimen for a longer period of time had less of a virologic response to T-1249. Although very preliminary in nature, this study potentially brings up the dilemma of maintaining unsuppressed patients on an enfuvirtide-containing regimen. To what degree will enfuvirtide resistance possibly convey cross-resistance to T-1249 or other fusion inhibitors? This is still unknown, but on the basis of this early observation, it is worth looking out for. Since T-1249 and other fusion inhibitors are not currently available and their clinical benefit is still in question, it might be premature to make any clinical decisions on the basis of these preliminary results.
Other Entry Inhibitors
Resistance data on the investigational CCR5 antagonist UK 427-857 were presented. A large panel of recombinant viruses with envelopes taken from various subtypes and with RT and protease-containing mutations were shown to be inhibited by the drug. This would suggest that specific HIV subtype or previous RT or protease use would not preclude the drug's activity. Another study showed preliminary data on a gp41 targeted lead compound, ADS-J1, that showed fusion inhibitor qualities. These data suggested this might be a new low-molecular-weight prototype agent worth pursuing.
References
The International HIV Drug Resistance Workshop: Basic Principles and Clinical Implications, now in its 12th year, is always an excellent opportunity to see new data regarding resistance to approved and investigational drugs. In this review I will summarize the key studies presented relating to protease and entry inhibitors and focus on their clinical relevance.
Protease Inhibitors
In reviewing the new and investigational protease inhibitors (PIs), let's consider these agents according to their stage of clinical development:
Atazanavir: Just approved by the FDA
Tipranavir: Large phase 3 clinical trials under way
TMC114: Early clinical trials under way
R0033-4649: Transitioning from preclinical to clinical development
P-1946: Compound in laboratory evaluation
Although many burning questions regarding atazanavir resistance remain, regrettably nothing was presented at the meeting. Key resistance questions are left open regarding this new drug: How commonly is the I50L mutation seen in large series of atazanavir-failing patients, and to what degree are other mutations associated with it? Is this the common mutation also seen in non-subtype B-infected patients? How does the resistance profile of ritonavir-boosted atazanavir differ (if at all) from the unboosted dose? How does boosted atazanavir compare with lopinavir/ritonavir and other boosted PIs in treating PI-experienced patients? The answers to all of these questions will have to be deferred until future meetings.
Tipranavir
Investigators presented a number of studies related to the resistance of tipranavir, an investigational PI now in large phase 3 clinical trials. Data from a study of PI-experienced patients receiving ritonavir-boosted tipranavir were analyzed. Baseline genotypes and drug susceptibility values were correlated with short-term HIV RNA changes to determine predictors of virologic success. The presence of 3 or more mutations at positions 33, 82, 84, or 90 was predictive of a poor response. These were characteristically accompanied by 16 or more additional PI-related mutations. An additional abstract identified L10I, K20M/L/T, M46I, I54V, and L63A/D/T as some of these additional mutations. The authors hypothesize that the additional mutations may be compensatory in nature due to the large impact 33, 82, and 84 have on the enzyme's active site. The term "UPAMs" (universal PI-associated mutations) was coined by the authors for the 4 key tipranavir-related mutations since they found them associated with cross-resistance to many other PIs. There was some controversy at the meeting regarding the appropriateness of the term. In attempting to delineate a clinically relevant cutoff for drug susceptibility, a baseline fold-change of greater than 2 was found to be associated with a poor tipranavir response.
So where do these findings put us with regard to tipranavir resistance? Common, known PI mutations impact tipranavir, and their accumulation results in increasing levels of resistance. Large numbers of mutations are often required to produce very small reductions in tipranavir susceptibility, and a number of key mutations have been identified. Similar mutational patterns often result in far greater reductions in susceptibility to other PIs, although the "clinical cutoff" of 2 for tipranavir is lower than that of some of the other boosted PIs. The absolute number of mutations required to incur a poor tipranavir response is impressive, and the need for multiple key mutations is also reassuring. Still, there is no clear-cut evidence of an advantage over currently approved boosted PIs, and we will have to await the results of the large comparative studies currently under way. Since these studies are being performed in PI-experienced patients with significant resistance, hopefully they will provide considerable insight into the clinical value of tipranavir in this population urgently in need of new drugs.
TMC114
Data on the experimental PI TMC114 were also presented. TMC114 had been studied in a short-term trial in which multiple PI-experienced patients received the drug at various doses, all with ritonavir boosting. After 14 days, TMC114/ritonavir demonstrated a 1.35-log reduction in HIV RNA. Baseline samples showed multiple PI resistance mutations (median 6), with the majority having more than 1 primary mutation. These viruses showed a good deal of reduced susceptibility to currently approved PIs. Their median fold-change in EC50, as compared with wildtype, was 1.8 for TMC114. The investigators were not able to report which of the mutations present had the greatest impact on TMC114 or which mutations characteristically develop after failing the drug. The results of this short, preliminary trial show promise, although much on the resistance characteristics of TMC114 will still need to be determined before we can evaluate how unique a drug it is. In addition, until a safe and well-tolerated dosage is determined and a clinically relevant cutoff defined, comparing TMC114 susceptibility data to data for other PIs provides little insight. A study comparing the crystal structures and thermodynamics of TMC114 to those of other PIssuggested the drug would have activity against multi-PI-resistant viruses. These favorable resistance characteristics would be the result of a tight interaction between the inhibitor and the enzyme and its relatively small volume that is contained within the substrate envelope. Additional groups using similar technologies also presented hypotheses regarding the development of new types of inhibitors with activity against highly mutated protease enzymes. It is hoped that these insights will lead to clinically useful next-generation PIs -- time will tell.
R0033-4649 and P-1946
Preclinical data from 2 additional investigational PI compounds were presented. Follow-up data regarding the previously described R0033-4649 showed activity against viruses containing multiple mutations. In addition, data regarding specific mutational patterns and reductions in susceptibility were shown that suggested good potential activity, if sufficiently high levels can safely be obtained in patients. P-1946, the prototype of a new class of potential PI inhibitors, was also shown to possess favorable resistance characteristics. It was comforting to see many potential new agents in the pipeline.
New PI Mutations
A number of new PI mutations were described, although not all of them appear to have a great impact on resistance. Two additional amino acid changes at the well-known resistance position 84 were described, and it appears that in addition to the known I84V variant, I84A and I84C both are significant mutations. Similarly, I47A joins I47V as a relevant mutation. L23I and K55R were also described but seem to have a small effect on resistance. Although all these new mutations are very rare, knowledge of them will help us further optimize our interpretation of resistance assay results.
Entry Inhibitors
New resistance data on entry inhibitors were limited at the meeting. I will first review new data presented on the recently approved fusion inhibitor enfuvirtide (T-20) and the follow-up compound T-1249, and then briefly mention other investigational entry inhibitors described.
Enfuvirtide (T-20) and T-1249
Laboratory susceptibility studies using different constructs of envelope fragments were performed to determine which areas held the determinants of enfuvirtide resistance. Interestingly, in addition to the HR1 region of gp41, the gp41 HR2 region was also found to influence susceptibility to the drug. A second study suggested some changes in gp120 might also be important. These findings provide additional support for the importance of the HR1 mutations found in previous studies in determining enfuvirtide resistance, but also open the door to continued research into additional areas that appear also to be involved. Further work looking at specific mutations in the gp41 HR2, as well as other sites contributing to fusion inhibitor resistance, will hopefully come soon. In the meantime, we should realize that our understanding of enfuvirtide resistance is not complete and is not yet ready for clinical implementation. Further study and possibly additional diagnostic tools are needed.
Investigators presented resistance data from the previously reported study of T-1249 in enfuvirtide-treated patients. Patients receiving an enfuvirtide-containing regimen, but with HIV RNA levels above 5000 copies/mL, were switched to T-1249 and showed a significant short-term reduction in viral load. Resistance analysis of these viruses at the time of switch showed multiple mutations in gp41 HR1 amino acids 35-45, as might be suspected on the basis of previous studies. In addition, very high levels of reduced susceptibility to enfuvirtide were seen (mean fold-change 150.1), with far more modest reductions to T-1249 (mean fold-change 1.8). A key clinical observation from this trial was the correlation between length of previous treatment on enfuvirtide and subsequent response to T-1249. Patients who had remained on a failing enfuvirtide-containing regimen for a longer period of time had less of a virologic response to T-1249. Although very preliminary in nature, this study potentially brings up the dilemma of maintaining unsuppressed patients on an enfuvirtide-containing regimen. To what degree will enfuvirtide resistance possibly convey cross-resistance to T-1249 or other fusion inhibitors? This is still unknown, but on the basis of this early observation, it is worth looking out for. Since T-1249 and other fusion inhibitors are not currently available and their clinical benefit is still in question, it might be premature to make any clinical decisions on the basis of these preliminary results.
Other Entry Inhibitors
Resistance data on the investigational CCR5 antagonist UK 427-857 were presented. A large panel of recombinant viruses with envelopes taken from various subtypes and with RT and protease-containing mutations were shown to be inhibited by the drug. This would suggest that specific HIV subtype or previous RT or protease use would not preclude the drug's activity. Another study showed preliminary data on a gp41 targeted lead compound, ADS-J1, that showed fusion inhibitor qualities. These data suggested this might be a new low-molecular-weight prototype agent worth pursuing.
References
McCallister S, Kohlbrenner V, Squires K, et al. Characterization of the impact of genotype, phenotype, and inhibitory quotient on antiviral activity of tipranavir in highly treatment-experienced patients. Antiviral Therapy. 2003;8:S15.
Hall D, McCallister S, Neubacher D, Kraft M, Mayers DL. Characterization of treatment-emergent resistance mutations in two phase II studies of tipranavir. Antiviral Therapy. 2003;8:S16.
De Meyer S, Peeters M, Jordens C, et al. TMC114, a potent next-generation protease inhibitor: characterization of antiviral activity in multiple protease inhibitor-experienced patients participating in a phase Iia study. Antiviral Therapy. 2003;8:S18.
King N, Prabu-Jeyabalan M, Wigerinck P, de Bethune MP, Schiffer CA. TMC114 binds within the substrate envelope of HIV-1 protease, which could account for its efficacy against multi-protease inhibitor-resistant virus. Antiviral Therapy. 2003;8:S19.
Kovari LC, Vickrey JF, Logsdon BC, et al. Crystal structure of a multidrug-resistant HIV-1 protease clinical isolate reveals an expanded active site cavity and represents a novel target for the design of protease inhibitors. Antiviral Therapy. 2003;8:S50.
Silva AM, Gulnik SV, Yu B, Eissenstat M, Erickson JW. Structural correlates of broad-sprectrum activity for a resistance-repellent HIV protease inhibitor. Antiviral Therapy. 2003;8:S51.
Heilek-Snyder G, Kohli A, Cammack N, Parkin N. In vitro cross-resistance profile of ROO33-4649 against a panel of multiply-substituted protease inhibitor-resistant viruses: role of common protease resistance mutations. Antiviral Therapy. 2003;8:S21.
Sevigny G, Tian B, Dubois, et al. Antiviral activity of P-1946, a novel anti-HIV protease inhibitor. Antiviral Therapy. 2003;8:S23.
Johnson E, Winters MA, Vyas K, Merigan TC, Shafer RW. Preliminary characterization of a newly described protease substrate cleft mutation at position 23. Antiviral Therapy. 2003;8:S53.
Kagan RM, Shenderovich M, Ramnarayan K, Heseltine PNR. Emergence of a novel lopinavir resistance mutation at codon 47 correlates with ARV utilization. Antiviral Therapy. 2003;8:S54.
Mo H, Parkin N, Stewart KD, et al. I84A and I84C mutations in protease confer high-level resistance to protease inihibitors and impair replication capacity. Antiviral Therapy. 2003;8:S56.
Morgan E, Pillay D, Cane P, et al. The HIV-1 protease mutation K55R is associated with the presence of the M46I/L mutation. Antiviral Therapy. 2003;8:S57.
Stanfield-Oakley SA, Jeffrey J, McDanal CB, et al. Determinants of susceptibility to enfuvirtide map to gp41 in enfuvirtide-naïve HIV-1.
Su C, Heilek-Snyder G, Fenger D, et al. Search for polymorphic sites in R5 tropic HIV-1 Env and enfivirtide drug susceptibility in baseline isolates from TORO 1 and TORO 2. Antiviral Therapy. 2003;8:S59.
Miralles GD, Melby T, DeMasi R, et al. Baseline and on-treatment gp41 genotype and susceptibility to enfuvirtide (ENF) and T-1249 in a 10-day study of T-1249 in patients failing an ENF-containing regimen (T1249-102). Antiviral Therapy. 2003;8:S24.
Westby M, Napier C, Mansfield R, et al. Sensitivity of env-gene recombinant viruses derived from antiretroviral drug-sensitive and -resistant HIV-1 clinical isolates to the novel CCR5 antagonist, UK-427,857. Antiviral Therapy. 2003;8:S26. 17. Armand-Ugon M, Gutierrez A, Jiang S, Clotet B, and Este JA. ADS-J1, a non-peptidic low molecular weight HIV fusion inhibitor targeting gp41, with no cross-drug resistance with peptidic HIV fusion inhibitors T-20 and C-34, and HIV binding inhibitors. Antiviral Therapy. 2003;8:S27.