Integrase
strand transfer inhibitors (INSTIs) are the newest class of
antiretroviral drugs to be approved for treatment and act by inhibiting
the essential HIV protein integrase from inserting the viral DNA genome
into the host cell’s chromatin. Three drugs of this class are currently
approved for use in HIV-positive individuals: raltegravir (RAL),
elvitegravir (EVG), and dolutegravir (DTG), while cabotegravir (CAB) and
bictegravir (BIC) are currently in clinical trials. RAL and EVG have
been successful in clinical settings but have relatively low genetic
barriers to resistance. Furthermore, they share a high degree of
cross-resistance, which necessitated the development of so-called
second-generation drugs of this class (DTG, CAB, and BIC) that could
retain activity against these resistant variants. In vitro selection
experiments have been instrumental to the clinical development of
INSTIs, however they cannot completely recapitulate the situation in an
HIV-positive individual. This review summarizes and compares all the
currently available information as it pertains to both in vitro and in
vivo selections with all five INSTIs, and the measured fold-changes in
resistance of resistant variants in in vitro assays. While the selection
of resistance substitutions in response to RAL and EVG bears high
similarity in patients as compared to laboratory studies, there is less
concurrence regarding the “second-generation” drugs of this class. This
highlights the unpredictability of HIV resistance to these inhibitors,
which is of concern as CAB and BIC proceed in their clinical
development.
Since
the beginning of the pandemic, HIV/AIDS has claimed the lives of over
35 million people, and approximately 35 million individuals are
currently infected [
1].
Highly active antiretroviral therapy (HAART) has transformed a positive
HIV diagnosis from a former death sentence into a chronic, manageable
disease. However, no cure yet exists for HIV and patients must remain on
therapy for the entirety of their lives which makes the development of
drug resistance in the virus a real concern. In fact, drug resistance
has been documented for every currently available drug class in patients
[
2].
This makes the continued study of the mechanisms of HIV drug resistance
and novel therapeutics a top priority for HIV scientists worldwide.
The reverse transcriptase (RT) enzyme of HIV is highly error-prone, introducing mutations into the genome at a rate of 1.4 × 10
−5 mutation per base pair, per replication cycle [
3].
This high mutation rate allows for the generation of multiple different
viruses within an infected individual, sometimes referred to as
“quasi-species.” If one of these quasi-species has a mutation that
provides a selective advantage for replication in the presence of
antiretrovirals (ARVs), it will out-compete other viral forms to become
the dominant species [
4].
The integrase (IN) enzyme catalyses the insertion of the viral DNA
(vDNA) into the host’s genome through two catalytic actions: 3′
processing and strand transfer. In the cytoplasm, IN self-associates
into tetramers on the newly reverse transcribed vDNA, where it catalyzes
the removal of the last two nucleotides from the 3′ ends of both
strands [
5].
In addition, IN can spontaneously form larger multimers that are
stabilized by the addition of allosteric integrase inhibitors, and
reciprocally destabilized in the presence of DNA [
6,
7,
8,
9,
10].
After nuclear translocation, IN associates with lens epithelium-derived
growth factor (LEDGF)/p75 and is directed to sites of open chromatin,
where it will initiate strand transfer, i.e. the nucleophilic attack of
the 3′ hydroxyl groups on the viral DNA on the nucleotide backbone of
the host DNA. The integration process is completed by host gap-repair
machinery, resulting in a 5 base-pair repeat that flanks each end of the
viral DNA [
11].
The integrase strand transfer inhibitor (INSTI) class of antiretroviral
drugs is the latest to be approved for treatment of HIV-positive
individuals. As their name suggest, INSTIs inhibit the second step
catalyzed by IN, i.e. strand transfer, through competitive binding to
the enzyme’s active site. INSTIs not only displace the 3′ end of the
vDNA from the active site, but also chelate the divalent cation (Mg
2+ or Mn
2+) that is required for IN enzymatic activity [
12].
There are currently three INSTIs approved for the treatment of HIV
infection: raltegravir (RAL), elvitegravir (EVG), and dolutegravir (DTG)
[
13]. Cabotegravir (CAB) and bictegravir (BIC) are newer INSTIs currently in clinical trials [
14,15)
Although highly efficacious in the management of HIV, both RAL and EVG
are susceptible to virological failure through the development of
resistance mutations. What is more, most of the changes that cause
resistance to RAL also cause resistance to EVG, and vice versa [
16].
This is, however, not the case with DTG. Not only does DTG appear to
have a higher genetic barrier to resistance than either of the other two
drugs, it has not yet been shown to definitively select for any
resistance-associated changes in treatment-naïve patients [
17].
Although two reports of potential emergence of resistance in
individuals treated with DTG in first line therapy recently appeared,
baseline IN was not sequenced in one of these cases, nor did the
supposed-emergent mutation lead to persistent virological failure while
DTG was still being used together with an optimized background regimen
containing rilpivirine (RPV), an NNRTI with a modest genetic barrier to
resistance [
18].
Specifically, initial TDF/FTC/DTG treatment was supplemented with
ritonavir-boosted darunavir following failure; the latter drug was
subsequently substituted with RPV for reason of diffuse erythoderma. The
second case reported transient emergence of a T97A substitution that
did not confer any resistance on its own against DTG in vitro and was
not observed at subsequent time points [
19].
Although it cannot be excluded that unambiguously documented cases of
emergent resistance mutations against first-line DTG will eventually be
reported, it is expected that this will be rare. This is supported by
the fact that despite dolutegravir being used by tens of thousands
treatment-naïve individuals in Europe and the USA, the abovementioned
two cases are the only known reports of potential primary de novo
resistance against this drug. There have also been rare cases of
treatment failure with resistance mutations in treatment-experienced but
INSTI-naïve patients, and, in this setting, DTG has most often selected
for the novel resistance substitution R263K [
20].
Other substitutions at residues E92, Q148 and N155, have been reported
when DTG monotherapy was used in treatment-experienced patients.
Primary resistance substitutions arise first in response to INSTI drug
pressure and cause a decrease in susceptibility at the expense of viral
fitness, most often through alterations to the enzyme’s active site
where the inhibitors bind [
16,
21].
Secondary resistance substitutions arise after continued drug pressure
and usually act to alleviate the negative effects of primary mutations,
and may also increase levels of INSTI resistance [
22,
23].
Some of these secondary changes are specific to a certain primary
resistance pathway, but many may be selected after several different
primary mutations.
Pre-clinical and in vitro studies have been instrumental in the
evaluation of novel therapeutic agents for the treatment of HIV
infection, however they do not always accurately predict clinical
outcomes for patients. Laboratory viral strains and cell lines, although
excellent scientific tools, can never recapitulate in vivo human
infections with 100% accuracy. In this review, we compare both the in
vitro selection and antiviral activity reported for drugs of the INSTI
class with the analogous data available from treated patients to assess
the predictive power of in vitro studies for INSTI clinical outcomes.
In
2004 a group of researchers at Merck & Co. reported on the efficacy
of the diketo acid (DKA)-based lead compound L-870812 against simian
immunodeficiency virus (SIV) in infected rhesus macaques [
24].
This led to the approval of the first INSTI, raltegravir, in 2007 for
treatment-experienced AIDS patients with multidrug resistance, and two
years later for treatment-naïve individuals as well [
25,
26].
In the 10 years since its first approval, RAL has been shown to be well
tolerated in the vast majority of patients, although it is does require
twice daily dosing. It displays a modest genetic barrier to resistance,
with the most common mutational pathways consisting of changes at
positions Y143, Q148, and N155 [
27].......( more information in the link)
EVG is a monoketo acid derivative that also demonstrated high specificity for inhibition of HIV IN strand transfer reactions [
77]. EVG was developed by Gilead Sciences and approved for use in HIV infected individuals in 2012 [
26].
Because EVG is processed by the cytochrome p450 enzyme CYP3A4/5, it
needs to be co-formulated with cobicistat to boost plasma
concentrations. This permits once daily dosing of EVG [
78].
It is evident from both Tables
3.........( More information in the link)
The
relatively low genetic barrier and high degree of cross-resistance
among the so called “first-generation” INSTIs RAL and EVG spurred
research into the chase for “second-generation” drugs of this class,
aimed at retaining efficacy against RAL/EVG resistant variants.
There
have been four candidate second-generation INSTIs to date.
DTG,
manufactured by ViiV-Healthcare and GlaxoSmithKline, was approved in
2013 for both treatment-naïve and—experienced patients and is
the only
second-generation INSTI to be approved to date [
79].
MK-2048 showed potent activity against most RAL/EVG resistant variants
and did not select for the same substitutions in tissue culture studies
but its clinical development was halted due to poor pharmacokinetics.
Both CAB and BIC are promising and both are currently in advanced
clinical trials [
15,
19,
50,
80].
( More info in the link...)
There are fewer reports on the resistance patterns of
CAB, a novel INSTI
under development at GlaxoSmithKline. CAB was developed concomitantly
with DTG and shares most of its structure; it has the potential to be
formulated as a long acting injectable for both pre-exposure prophylaxis
and treatment of HIV infection [
84].
In the LATTE clinical trial, one patient in the CAB arm did develop a
mutation in the Q148 pathway, which suggests that this second-generation
INSTI may select for the same mutations as RAL and EVG [
15].
In in vitro selection studies, CAB has selected for changes at
positions 146 and 153 that could also be selected in the presence of EVG
and DTG, respectively (Table
4).
(More info in the link)
