ISSUE 27 - 18th July 1997
Sourced, Compiled and Edited by Paul Blanchard
Medical Advisor - Dr Graeme Moyle
, Chelsea & Westminster Hospital.
HIV DRUG RESISTANCE AND TREATMENT STRATEGIES: PART 1 |
The annual resistance meeting has evolved to be one of the most important and eagerly awaited conferences of the
year, particularly when, as in 1987, there is no International AIDS Conference. This year, for the first time, the meeting consisted
of the merger of three previously separate events, the International Conference on Drug Therapy in HIV Infection, the HIV
Drug Resistance Workshop and the Workshop on HIV Eradication. Attendance is strictly limited to approximately 200 leaders in the
field to maintain an informal workshop environment. Delegates from the UK included AIDS Treatment Project Executive Director Raffi
Babakhanian and leading clinicians Graeme Moyle, Mike Youle, Ian Weller and Ian Williams.
This issue of the Doctor Fax will examine conference presentations focusing
on drug resistance. Part 2 of this report will
look at presentations validating viral load as a surrogate marker, durability
of viral suppression and implications for treatment approaches.
Science Editorial Pessimistic Over Challenges Presented by Therapy for HIV-Infection |
ST. PETERSBURG, FLORIDA--When the media last year trumpeted the great advances being
made against HIV, speculating about cures and even the end of AIDS, the researchers' own caveats tended to get drowned out. But
at this meeting held here, emerging data about treatment failures sounded a discordant note that was hard to miss: While powerful
new drug combinations are delaying disease and death, they have serious limitations--and clinicians and patients who ignore
these shortcomings do so at their peril.
More than 200 leading AIDS researchers from around the world gathered here from 25
to 28 June for the workshop, which focused on HIV drug resistance, treatment strategies, and the possibility of eradicating the
virus from an infected person. One presentation after another reinforced the message that keeping HIV at bay, even with the most
potent three-drug cocktails now available, remains a daunting challenge. "Triple combination therapy can fail for a variety of
reasons," said John Mellors of the University of Pittsburgh Medical Center, one of the meeting's organisers. As this reality sets
in, infected people may end up feeling that their hopes were raised too high last year, he warned. "The pendulum will swing
back."
The meeting went into fine detail about why these failures occur. New, more sensitive
assays that measure levels of HIV indicate that even the best treatments have a difficult time completely suppressing viral
replication, which gives drug-resistant mutants a chance to appear. Less surprisingly, many treatments also fail because patients
don't "comply" with therapies that require taking dozens of pills--many of which have serious side effects and dietary restrictions--each
day. Although there were encouraging findings about new treatments allowing the immune system to recover if the virus
can be suppressed, researchers spelled out just how distant the goal is of completely rebuilding a full range of immune responses
in an HIV-damaged body
Last year's surge in hope was driven by dramatic findings about the wallop delivered
by combinations of two drugs that attack HIV's reverse transcriptase (RT) enzyme with one drug from a newer class of compounds
directed at the virus's protease enzyme. Several studies showed that such triple combinations--and even some cocktails of RT inhibitors
alone--can drive the amount of HIV in a person's blood, the "viral load," down so low that the most sensitive tests could
not detect any virus for more than a year in many patients. Researchers warned, however, that just because HIV couldn't be found
didn't mean it wasn't there--nor did it mean the virus wasn't replicating.
One of the more disconcerting findings reported here is that, just as researchers
feared, the "undetectable" HIV reported last year can routinely be detected with a more sensitive test. A year ago, the most sensitive
tests could measure viral levels down to 500 copies of HIV per millilitre of blood. New tests now measure as few as 20 copies
per millilitre. Brian Conway, of Vancouver, Canada's BC Centre of Excellence in HIV/AIDS, used such a test in a 151-person
study comparing two RT inhibitors to three RT inhibitors. One year after treatment began, 27% of the patients receiving one of the
two-drug combos had fewer than 400 copies of HIV. But when the samples were reanalysed with an assay that went down to 20 copies,
only 12% had "unquantifiable" levels. And when Conway ran the test on one sample below 20, he detected the virus three out
of 11 times. "A lot of people hear 'unquantifiable,' and they think 'zero,' " said Conway.
John Coffin of Tufts University in Medford, Massachusetts, suggested that researchers
consider changing the focus to the number of virally infected cells, which estimates suggest is about 1000 times higher than
the HIV copy number detected. So 20 copies, noted Coffin, would equal 20,000 infected cells. "If you say a person has less than
20,000 infected cells, it might give even the most optimistic patient pause," said Coffin.
Considering that many patients have tens of thousands of copies of HIV when they begin
therapy, a drop to, say, 400 copies is hardly bad news. The problem, however, is that if HIV is detected, it's replicating
and can mutate into resistant strains. Dale Kempf of Abbott Laboratories near Chicago, maker of the protease inhibitor ritonavir,
underscored this point. Kempf presented a study that analysed patients who had failed various ritonavir regimens--including triple
drug combos--because viruses resistant to the drug had emerged. He found that the most telling gauge of whether treatment would
eventually fail is how low a person's viral load fell. If the therapy knocked HIV down to 200 to 1000 copies per millilitre,
viral levels would rise again, on average, 128 days after treatment began. In contrast, for people whose viral levels went below
200--the limit of the assay Abbott used--treatment failed in 199 days. "Very low viral load must be achieved to ensure a durable
response, and we don't think 200 is low enough," concluded Kempf.
Douglas Mayers of the Naval Medical Research Institute in Bethesda, Maryland, added
yet another eye-opening finding about drug failures, by showing that they often have nothing to do with resistance. Mayers's genetic
analysis of HIV from 37 patients who failed various RT-protease inhibitor combinations revealed that 22% had no mutations
that would make them resistant to any of the drugs. Another 24% were resistant to RTs but not protease inhibitors.
Patients who fail one therapy can often switch to other combos of the 11 anti-HIV
drugs now on the market. Unfortunately, however, many mutations that confer resistance to one drug will render similar drugs useless.
The best hope for people for whom several drugs have failed is new drugs that attack novel HIV targets. But there is a dearth
of such drugs in the pipeline, says meeting co-organiser Charles Boucher of University Hospital in Utrecht, the Netherlands.
"I'm confident that [these drugs] are not going to be developed in the next 3 to 4 years," says Boucher. "It's going to be a disaster."
Other clinicians at the meeting warned that they already are running out of options
for many of their patients who have tried and failed several drug regimens. "I think three-drug therapy is an incredible advance,
but our ability to treat people who fail is troubling," said Margaret Fischl of the University of Miami School of Medicine.
"We don't have anything to offer them." So in spite of the vast improvement in treatment that the new therapies have brought, there
were no trumpets playing victory tunes here.
Ref: Science 4 July 1997; 277 (5322):32 (in News)
Source: Aegis
|
As potent as the latest antiretroviral agents might be, they are still imperfect approaches
to the treatment of HIV. Viral evolution and drug resistance suggest that physicians and patients embarking on treatment
with currently available antiretrovirals should try to maximise viral suppression with their first choice of combination, and
to monitor frequently (some even say monthly) to allow the first signs of resistance to be acted upon swiftly. Further caution
may also be warranted over stable HIV-infected patients being urged to consider treatment. |
In perhaps the most controversial presentation of the meeting, Eric Arts, Case Western
Reserve University, suggested that zidovudine (ZDV) treatment appears to stimulate reverse transcription, and may confer cross-resistance
to other nucleoside analogues. Reverse transcription was measured by quantitative assays for viral DNA synthesis
in vitro. Cells infected with ZDV-resistant virus strains which have the T215Y mutation, but not the M41L or K70R mutation, had
high levels of full length HIV DNA synthesis. This stimulation of reverse transcription may be up to one hundred fold, and lead
to resistance to didanosine (ddI) and zalcitabine (ddC) even in the absence of specific mutations associated with resistance to
these two drugs when used as monotherapy. If confirmed, these results suggests that the use of ZDV in combination therapies may
be detrimental to the success of therapy with other nucleoside reverse transcriptase (RT) inhibitor analogues. The specific mechanism
for this phenomenon is not known, but this may be mediated through other enzymatic activities. Theoretically, this enhanced
activity may allow HIV to overcome the inhibitory effects of ZDV and perhaps all reverse transcriptase inhibitors.
Ref: [Abstract 10]
|
Comment: This intriguing study suggests that ZDV metabolites may stimulate reverse
transcriptase activity. If true, this could lead to cross-resistance to all reverse transcriptase inhibitors. This may explain
the lack of ddI and ddC mutations in patients who fail the combination of ZDV plus ddI or ZDV plus ddC. |
Genetic resistance for combination reverse transcriptase inhibitor therapy may be
complex and occur through distinct pathways. Phenotypic assays show that a significant percentage of the viral isolates from patients
failing ZDV/3TC therapy were resistant to both zidovudine and lamivudine. Zidovudine (ZDV) selects for several mutations,
including changes at positions 215 and 41. Lamivudine selects for a mutation at position 184. This M184V mutation appears to suppress
the effects of 215 and 41, thus allowing continued ZDV activity. This observation, confirmed in clinical trials, provides
the basis for the popular ZDV/3TC combination.
Resistance to the combination of ZDV and 3TC can occur. Until recently, the genetic
basis for this dual resistance was unknown. This group from Glaxo-Wellcome presented preliminary data suggesting that known polymorphisms
at positions 211 and 214 may confer some degree of dual ZDV/3TC resistance. When these polymorphisms are present, the
M184V mutation is no longer able to restore sensitivity to ZDV. A novel mutation at position 333 (G333E) of the reverse transcriptase
gene may also facilitate cross-resistance via a distinct pathway [Kemp, abstract 11
|
Comment: Complex patterns involving these mutations, and others, probably provides the genetic basis for dual resistance to ZDV/3TC. This study again illustrates the limitations of our current knowledge regarding
the genetic basis of drug resistance. |
HIV-1 Protease Inhibitors and Cross Resistance |
The bad news from the conference was the growing recognition of generalised resistance
to all protease inhibitors as a class. It appears that specific mutations associated with resistance to a particular protease
inhibitor can be selected for during treatment with other ones.
The HIV-1 protease is a fairly small molecule, with only 99 amino acids. At least
eleven amino acid changes have been found to contribute to protease inhibitor resistance. Although a few mutation patterns were
thought to be specific for certain protease inhibitors (e.g. G48V and L90M for saquinavir resistance, and D30N for nelfinavir resistance)
it appears that all of the protease inhibitors can select for multiple mutations. In addition, the more protease inhibitor
mutations there are, the more cross resistance there is.
Ronald Swanstrum, University of North Carolina at Chapel Hill, used increasing concentrations
of various protease inhibitors in vitro to select for resistant HIV-1 strains, and assessed the ability of other protease
inhibitors to inhibit these resistant strains [Swanstrum et al. abstract 15
]. At higher drug concentrations, i.e., higher selective pressure, greater the numbers
of protease inhibitor resistant mutations were seen. These experiments were formed in step wise manner for saquinavir, ritonavir,
and indinavir. In all cases, cross resistance to the other two inhibitors was similar to that seen to the protease inhibitor
used for selection of the resistant strains. These experiments support concerns that resistance to any single protease inhibitor
will affect subsequent regimens using other drugs in this class.
|
Comment: A general theme at the conference was that one protease inhibitor selects
for resistance to the entire class of drugs. Swanstroms in vitro work confirms these in vivo observations, and suggests that each inhibitor
selects for very similar mutation patterns. Notably, nelfinavir was not used in this study. |
The effect of sequential protease inhibitor treatment on resistance was examined by
Mark Winters of Stanford Medical Center [Winters et al. abstract 17
]. Genetic correlates of protease inhibitor resistance were assessed in patients who
had received 1800, 3600, or 7200 milligrams per day of saquinavir. The saquinavir therapy was given to 40 antiretroviral-naive
patients, having CD4 lymphocyte counts ranging from 188 to 527/mm3, and to 16 saquinavir-experienced patients having CD4 lymphocyte
counts ranging from 21 to 306/mm3. The analysis focused on 10 patients who failed long-term (1-2 years) saquinavir therapy.
The predominant mutation selected for resistance to saquinavir was the G48V and L90M mutations. The G48V occurred more frequently
in patients treated with high dose saquinavir. A significant percentage of patients (35%) also had mutations typically associated
with other protease inhibitors, including M36I, M46I, V82A, and I84A.
All patients (6 of 6) who developed G48V eventually developed V82A (a mutation associated
with resistance to indinavir or ritonavir), either while on continued saquinavir or after switching to nelfinavir or indinavir.
V82A did not typically develop in the presence of L90M. All six patients exhibiting the G48V mutation associated with saquinavir
resistance acquired the V82A mutation associated with indinavir/ritonavir treatment. The D30N mutation associated with
nelfinavir treatment was not seen.
|
Comment: The development of complex mutation patterns was common in patients after
prolonged (> 1 year) treatment with saquinavir. In such patients, cross-resistance to other protease inhibitors would be expected.
Preliminary data from this presentation, and others, suggests that the resistance pathway taken by the virus may be influenced
by the initial presence of L90M vs. G48V. |
Eastman and colleagues performed genotypic assays on 7 patients who initially responded
to and eventually failed saquinavir (hard gel capsule). Viral rebound was associated with the development of an L90M mutation
alone (n=4), G48V alone (n=2) or both (n=1). All 3 subjects with the G48V mutation developed subsequent additional mutations,
including the V82A mutation typically seen with indinavir or ritonavir resistance. Two subjects acquired an L90M mutation that
was subsequently lost in the presence of the V82A mutation [Eastman et al, abstract 30
].
In a companion presentation, 13 patients added an experimental formulation of saquinavir
(saquinavir-soft gel capsule) to a stable nucleoside analogue regimen. After 8 weeks of therapy, a potent 1.7 log decrease
in plasma HIV-1 RNA was observed. Six patients subsequently failed (defined as a viral rebound of > 1 log). Viral rebound was
associated with L90M (n=1), G48V (n=1) or both (n=2). One patient rebounded with V77E only (results from the sixth patient were
not available). Interestingly, one patient with prior ritonavir failure switched to saquinavir soft-gel capsule. At the time of
the switch, this patient had three mutations associated with high-level resistance to indinavir and ritonavir (mixed V82V/A, M46M/L
and I84I/V). As expected, SQV-SGC selected for L90M, but V82, M46 and I84 all reverted to wild-type. This patient subsequently
switched to indinavir, and has had a durable 7 month response, despite the prior presence of mutations highly correlated with
indinavir resistance [Deeks et al, abstract 69
].
|
Comment: These two studies confirm that G48V and L90M are the two most important mutations
associated with saquinavir therapy. They also suggest that L90M may not be compatible with V82A, particularly when other
so-called "compensatory mutations" are not present [see also Winters, abstract 17
]. |
In a companion study to Winters et al (abstract 17
), Jody Lawrence from Stanford reported on 16 patients who switched to nelfinavir
after failing saquinavir. When possible, nucleoside analogues were modified. Patients switched to nelfinavir after a mean duration
on saquinavir of 11 months.
After 4 weeks of therapy with nelfinavir, the mean decrease in HIV-1 RNA was 0.56
log. Only 2 of 16 subjects had a > 0.5 log decline through week 12 of nelfinavir therapy. At week 12, 2 patients failing nelfinavir
had evidence of D30N, the classic mutation associated with nelfinavir resistance. Most patients who failed nelfinavir had mutations
typically seen with other protease inhibitors.
Considering the poor response to nelfinavir, the study was modified. Ten patients
who failed nelfinavir were switched to indinavir plus nevirapine. After 4 weeks of therapy, the mean reduction in viral load was
1.8 log. The durability of this potent response is unknown [Lawrence et al, abstract 64
].
|
Comment: In this study, saquinavir resistance conferred cross-resistance to nelfinavir.
Therefore, the utility of nelfinavir in patients with prolonged prior saquinavir may be limited. On the other hand, the combination
of indinavir plus nevirapine appeared to be very active over 4 weeks of follow-up, despite the fact that these patients
had failed two protease inhibitors. The long-term efficacy of indinavir plus nevirapine as salvage therapy is unknown. Finally,
resistance patterns to a particular protease inhibitor can be very different when the drug is used in patients who have already
failed a first-line protease inhibitor. |
Sequential protease inhibitor therapy was a common theme at this years conference. In this study, 54 patients were treated with saquinavir and followed
prospectively. Twenty-two of these patients eventually failed saquinavir and were switched to indinavir. Ten of the 22 patients
had a durable response to indinavir, while 12 eventually failed. Failure was defined as having a viral load greater than 3.5 log
copies/mL after a mean 4 month period of treatment with indinavir.
Genotypic analysis was performed on the 12 patients who failed indinavir. Prior to
the introduction of indinavir, 5 of the 12 patients had a mutation at position L90M. All 5 patients maintained L90M in the presence
of indinavir; none developed the classic indinavir associated mutation V82A. A previously uncharacterised mutation, G73S, was
observed in all post-indinavir specimens.
In the other 6 patients who failed indinavir after prolonged saquinavir therapy, saquinavir
related resistance patterns were not present at the time of the switch. Surprisingly, after switching to indinavir, all
patients failed with typical saquinavir related mutations (L90M or G48V). Typical indinavir related mutations, such as V82A, were
rare [Dulioust et al, abstract 16
].
|
Comment: These results suggest that saquinavir related genotypic resistance confers
cross-resistance to indinavir in vivo, despite the lack of significant cross-resistance in vitro. Furthermore, patients may have
a normal viral genotype (i.e., no mutations evident on genotypic analysis) after prolonged therapy with saquinavir, yet still
rapidly select for saquinavir related mutations in the presence of indinavir. This suggests that these mutants pre-existed below
the levels detectable with current genotypic assays. |
The combination of ritonavir and saquinavir is extremely effective in suppressing
HIV-1 replication. In an ongoing study of this combination, approximately 80 to 90% of subjects have had a durable virologic response.
In this study, genotypic analysis was performed on viral isolates from five patients who exhibited a rebound in viral load.
Rebound was associated with the emergence of V82A and I54V in all five patients. I84V and M36I were also seen. L90M and G48V,
both typically seen with saquinavir failure, were not observed [Molla et al, abstract 83
].
|
Comment: Patients who fail the combination of ritonavir and saquinavir appear to select
primarily for ritonavir related mutations. Since ritonavir resistance is known to confer cross-resistance to saquinavir, this
observation is perhaps not surprising. More resistance data on this combination is needed. |
Genotypic analysis of HIV from patients treated with nelfinavir-containing combinations
for up to one year were presented by Amy Patick from Agouron Pharmaceuticals in San Diego [Patick et al. abstract 18
]. Patients who failed this regimen were found to predominately have the D30N mutation,
without detection of amino acid changes associated with ritonavir or indinavir resistance. The L90M mutation associated with
saquinavir resistance was only rarely observed. This may be related to the sensitivity of assays for genotypic resistance detection,
since in other presentations, it was observed that selection for protease inhibitor mutations not specific for resistance
to the inducing drug may occur at fairly low abundance and may not be detected until increased selective pressure is exerted by
starting the new therapy. Subsequent therapies may also result in increased abundance of mutations selected for in initial therapies.
In several studies the L90M mutation associated with saquinavir resistance was often not seen in patients failing saquinavir
until additional selection pressure was provided by subsequent therapy with indinavir. Nonetheless, the order of protease inhibitor
use will certainly influence the pathway toward resistance by providing higher selective pressure for certain protease
inhibitor mutations.
Several groups reported cross-sectional results from patients failing combination
therapies.
One group, studying viral isolates from the military health care system, reported
that 21% of patients failing a combination regimen had no evidence of genotypic resistance. Surprisingly, over 50% of patients failing
a protease inhibitor-containing regimen had no genotypic changes in the protease gene [Mayers et al, abstract 80
].
In a study of patients in the Denver area, 33 patients failing a protease inhibitor
containing regimen were analysed (most patients had received indinavir). Compared to the clade B consensus sequence, patients
failing combination therapy had a mean of 4 mutations in their protease gene. Four of the 33 subjects had no protease mutations.
Despite the infrequent use of saquinavir, L90M was very common (15 of 37 isolates) [Young et al, abstract 65
].
|
Comment: The clinical experience with protease inhibitors in the primary care setting is likely to be very different from the experience seen in controlled clinical trials. These studies illustrate this point. The observation that many patients fail without genotypic evidence of resistance may be due to several factors: (1) variable drug absorption, (2) enhanced drug metabolism, (3) poor adherence to the drug regimen and/or (4) limited sensitivity of the genotypic
assays.
|
Unfortunately, it appears likely that once a genetic pathway to resistance has been
started, there may be no going back. John Condra of Merck Laboratories presented data of a patient in whom indinavir resistance
developed after nine months of monotherapy10. Despite cessation of drugs for one year and loss of detectable protease inhibitor
resistance mutations in circulating virus, resistant virus emerged within three weeks of reinstitution of indinavir therapy after
one year off indinavir therapy. In heavily pre-treated patients, multi-drug resistant viruses are present in both the RT and
protease genes creating a situation in which there is no combination of antiretroviral drugs which will effectively inhibit HIV
replication11. Such patients, unfortunately, are not rare, and there may be no treatment options available for suppression of virus
in these situations.
References: All abstract numbers refer to the: Program and abstracts of the International
Workshop on HIV Resistance, Treatment Strategies, and Eradication; St. Petersburg FL, USA, June 25-28, 1997.
Source: Meeting reports by Raffi Babakhanian AIDS Treatment Project, Steven G Deeks
(HIV InSite) and William OBrien (healthcg.com).
|
OTHER NEWS
|
EATG Secures Expanded Supplies of Nelfinavir for Europe
After months of concern amongst European treatment activists and physicians over the
supply of nelfinavir for compassionate use, Agouron, the drugs manufacturer, has pledged 2000 dosages. This figure will increase
to 5400 doses by January 98 and 1088 paediatric dosages of nelfinavir powder will also be supplied as from
now. The supply of nelfinavir for clinical trials in Europe has also been promised so that long postponed trials can begin.
This welcome development comes after a bitter dispute between the European AIDS Treatment
Group (EATG) and Agouron which also involved Roche Pharmaceuticals, the European Marketing partner for the protease inhibitor.
The long running dispute centred on the lack of access for compassionate use of nelfinavir in Europe long after the drugs
approval in the US and its free availability in US pharmacies.
Earlier in the month an action letter was sent to the FDA by the Executive Director
of the EATG, Arjen Broekhuizen concerning Agourons supply difficulties which sparked meetings between the drug company, Roche
and the EATG, including an emergency meeting at the St. Petersburg conference between Agouron and Raffi Babakhanian, EATG Director
and Executive Director of AIDS Treatment Project.
Accessing nelfinavir in the UK for compassionate use on a named patient basis was
particularly problematic, with a waiting list building up. The current 70 places for the UK should be scaled up to 100 immediately
and to 300 next month, hopefully eliminating any unacceptable wait.
| Clinicians and activists in Europe will be monitoring the availability of nelfinavir in the coming weeks and would ask any doctor who is having difficulties accessing this drug to contact AIDS Treatment Project on +44 (0) 171 793 7444 or by fax +44 (0) 171 793 8300. |