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Dear all,
For nucleoside analog drugs, the FDA guidance on Antiviral Product
Development recommends "that sponsors determine the intracellular
half- life (t1/2) of the triphosphate form of the active drug moiety
in stationary and dividing cells from the target tissue." The
information is apparently expected in an NDA filing, but I wasn't sure
if the FDA expects intracellular half-life data as part of an IND
package. Does anyone have experience with this?
Thanks in advance,
Cynthia
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Cynthia Sung remarked:
'For nucleoside analog drugs, the FDA guidance on Antiviral Product
Development recommends "that sponsors determine the intracellular
half- life (t1/2) of the triphosphate form of the active drug moiety
in stationary and dividing cells from the target tissue." '
This appears to be a quote from http://www.fda.gov/Cder/guidance/7070fnl.htm
..
The context is a discussion of the EC50. "The effective concentration
is the concentration of product at which virus replication is
inhibited by 50 percent (e.g., EC50 for cell-based assays; IC50 for
biochemical or subcellular assays). ...It is important that the
effective concentration be consistent with data supporting the
mechanism of action. An investigational product that inhibits virus
replication at concentrations lower than biochemical data for the
proposed mechanism indicates that another target or mechanism of
inhibition may be affected."
The recommendation about determining the intracellular half-life seems
to be a non-sequitur (i.e. does not follow from the preceding
material). Cynthia, can you or anyone else point to any discussion by
FDA of the relevance of the intra-cellular t1/2 is to the
understanding of the science or the clinical use of these drugs?
Is there any direct evidence that intra-cellular triphosphate concs
are any more predictive of drug effects than plasma concentrations? As
far as I am aware there is no evidence for this for the intra-cellular
triphosphate metabolite of gemcitabine (see Tham et al. 2008 a,b)
Nick
1. Tham LS, Wang L, Soo RA, Lee SC, Lee HS, Yong WP, et al. A
pharmacodynamic model for the time course of tumor shrinkage by
gemcitabine + carboplatin in non-small cell lung cancer patients. Clin
Cancer Res. 2008a;14(13):4213-8.
2. Tham LS, Wang LZ, Soo RA, Lee HS, Lee SC, Goh BC, et al. Does
saturable formation of gemcitabine triphosphate occur in patientsb?
Cancer Chemother Pharmacol. 2008.
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.aaa.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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The following message was posted to: PharmPK
Hi Nick,
A review article about antiretroviral therapy by PJ Piliero* asserts
that intracellular half-life is the relevant parameter for appropriate
selection of frequency of drug administration. The discussion centers
around being able to support once-a-day dosing (highly advantageous
for patient compliance with multi drug regimens of HIV). I suppose
what really matters is whether the intracellular concentration during
a dosing interval remains above some key concentration like an IC50 or
Ki. When I looked up the prescribing information for approved
nucleosides, information on intracellular half-life is provided for
most of them. Interestingly, the intracellular triphosphate half-life
is frequently quite a bit longer than the plasma half life of the
parent compound Given this, and the recommendation in the FDA
guidance, I anticipate that a regulatory review will expect this
information at some point in drug development. I just wonder if its
necessary at the time of IND. Would appreciate comments from those who
have submitted packages to regulatory authorities on nucleosides.
Cynthia
*PJ Piliero, Pharmacokinetc properties of nucleoside/nucleotide
reverse transcriptase inhibitors. J AIDS (2004); 37 Suppl 1, S2-S12
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The following message was posted to: PharmPK
Cynthia, Nick and others:
I have looked up the paper Nick mentioned in his prior post from
Clinical Cancer Research, and there is a problem that neither of the
two of you has addressed: is the concentration of drugs and their
metabolites in cells circulating in plasma a valid surrogate of the
concentration and the metabolism of such drugs at the TUMOR (target)
tissue? I do not believe we can make that assumption without
validation if we are talking about solid tumors.
For one, uptake of drugs into all cells is determined primarily by the
expression of the membrane transporters. There is only one set of
membranes between plasma and the inside of white cells, whereas in the
tumor drugs need to transfer from plasma to the interstitial fluid
space and from there into the tumor cells. This entails at least three
compartments, not two.
While you all know by now that I will bring up noninvasive imaging as
the tool of choice (and by the way, in our work we use the 19F signal
of gemcitabine and its metabolites to measure the target pK of these
agents using 19F-Magnetic Resonance Spectroscopy), it would be
wonderful if we could depend on more directly measurable sites - eg,
white cells. But before that can be accepted as relevant to what
happens in a solid tumor, there needs to be PROOF that this is the case.
I am cc'ing Bill Plunket in case he wishes to chime in, and because
the prior messages by Cynthia and Nick are not included here, I will
forward them to him.
Walter
--
Professor Walter Wolf, Ph.D. Distinguished Professor of Pharmaceutical
Sciences
Director, Pharmacokinetic Imaging Program
Department of Pharmaceutical Sciences, School of Pharmacy
Chair, Biomedical Imaging Science Initiative
University of Southern California 1985 Zonal Ave., Los Angeles, CA
90089-9121
E-Mail: wwolfw.-a-.usc.edu
http://www.usc.edu/research/initiatives/bisi/
http://www.macnis.org/
http://www.usc.edu/schools/pharmacy/faculty_directory/detail.php?id=59
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Cynthia,
I agree with you that what is important is to know the target
concentration and also to understand the time course of concentration
that will produce the desired effect. Unfortunately, the FDA antiviral
guidance is silent on these critical issues (as is the FDA Exposure
Response guidance).
We know however from other areas that half-lives alone are themselves
poor predictors of suitable dosing regimens. The half-life (plasma or
intra-cellular) as a predictor of a suitable dosing interval is
typically based on the naive assumptions of an immediate drug effect
and a linear conc-response relationship. Based on more mechanistic
concepts of delays in response and non-linear PD it is however
possible to predict the time course of drug effects from plasma
concentrations even if it clearly the case that some intracellular
mediator is on the causal path to the observed response e.g. warfarin
anti-coagulant effects. Dosing regimens can then be designed based on
these PKPD relationships which do not rely solely on the plasma half-
life.
The reference you provided (Piliero 2004) to support the use of
intracellular concs instead of plasma concs makes this assertion:
"Since NRTIs require intracellular activation, it has been
hypothesized that the intracellular level of the active triphosphate
metabolite of NRTIs might be a better predictor of virologic
effectiveness than plasma drug levels. This hypothesis has recently
been proved by the demonstration that intracellular concentrations of
the active triphosphate metabolite of 3TC and d4T, rather than levels
of unchanged drugs in plasma, correlated with virologic response in
HIV-infected patients.9"
The cited reference number 9 (Solas et al. 1998) which "proved" the
hypothesis is a case report of a single patient with measurements of
3TC-triphosphate intracellular concs but neither the plasma concs nor
the virologic response are reported !
Thus this paper does not provide evidence that one should accept
intracellular concs as being superior to plasma concs as being better
predictors of outcome. It also remains to be seen what the target
concentration profile (plasma or intracellular) would be. If anyone
knows where the evidence is for both of these points then please let
PharmPK readers know.
It would be especially valuable if someone from the FDA
pharmacometrics group would respond by revealing the Agency thinking
behind the anti-viral product guidance which recommends intra-cellular
half-lives as being important without any explicit connection to
target concentrations or target concentration profiles.
Please be aware that I am not challenging the elegant biochemical
studies that establish the mechanism of action of anti-viral drugs. I
have no expertise in that area. But I am challenging the dogma that
intra-cellular concentrations and half-lives are themselves more
informative than plasma concentrations as predictors of outcome via
appropriate PKPD models. If someone can provide the evidence then I
would be very happy to learn about this.
Nick
1. FDA. Guidance for Industry Exposure-Response Relationships --
Study Design, Data Analysis, and Regulatory Applications http://www.fda.gov/cder/guidance/5341fnl.htm
.. 2003.
2. FDA. Guidance on Antiviral Product Development -- Conducting and
Submitting Virology Studies to the Agency
http://www.fda.gov/Cder/guidance/7070fnl.htm. 2006.
3. Piliero PJ. Pharmacokinetic properties of nucleoside/nucleotide
reverse transcriptase inhibitors. J Acquir Immune Defic Syndr. 2004;37
Suppl 1:S2-S12.
4. Solas C, Li YF, Xie MY, Sommadossi JP, Zhou XJ. Intracellular
nucleotides of (-)-2',3'-deoxy-3'-thiacytidine in peripheral blood
mononuclear cells of a patient infected with human immunodeficiency
virus. Antimicrob Agents Chemother. 1998;42(11):2989-95.
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.at.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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The following message was posted to: PharmPK
Hi Nick et al.,
I'd like to make you aware of a recent paper in the Journal of Clinical
Oncology (Peter Galettis is the corresponding author) VOLUME 25 NUMBER
36 DECEMBER 20 2007:5704-5709. Randomized Crossover Study Evaluating the
Effect of Gemcitabine Infusion Dose Rate: Evidence of Auto-Induction of
Gemcitabine Accumulation.
Quote:
Patients with relatively greater levels of gemcitabine-triphosphate in
WBCs tended to have less under-dosing and a greater reduction in
midcycle neutrophils. However, this observation did not correlate with
plasma gemcitabine levels.
Best regards,
Frederik Pruijn
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Walter,
Thanks for taking the bait. I have in fact debated this issue face to
face with Bill Plunket and I look forward to any further comments he
may have. I am sure you know his work has been in what are called
"liquid" tumours (i.e. leukaemias) where the target cells of interest
may be directly studied. The problem indeed for solid tumours is being
able to get to the target cell in order to show that other more
conveniently located cells (white cells) share similar kinetics.
The studies reported by Tham et al. (2008a) used circulating white
cells as the site of intracellular conc measurements because of the
inaccessibility of tumour cells (non-small cell lung) for repeated
direct measurement. This problem is recognized in the Discussion "it
is still unknown whether the pharmacokinetics of dFdCTP in white cells
can be extrapolated to tumor or even normal tissue cells". I do no
wish to deny that intracellular dFdCTP is a mediator of the the
undoubted effects of gemcitabine. However, the report by Tham et al
asserts that there is no evidence based on white cell concs for using
this measurement as a biomarker to predict tumour response.
With regard to transporters -- I dont follow your argument about
tumours and white cells. There is still only one membrane for the drug
to cross. Transfer of drug from plasma to interstitial fluid is
primarily a diffusion process and not a membrane transporter process.
Tham et al. (2008b) also examined the dogma that such transporters are
saturable at clinically used concs of gemcitabine but could find no
evidence in the white cell. While I am fully willing to accept that
any transporter must eventually reach its maximum the issue is whether
this is relevant at concentrations that are known to be effective in
shrinking tumours. As far as I know there is no direct evidence of
saturability of gemcitabine transport into tumour cells in patients
undergoing treatment.
You ask for PROOF. Do you have proof that non-invasive, non-specific
measurements of gemcitabine and metabolites in the anatomical region
of a tumour such as you mention have any relationship to tumour
response? Where your studies able to show any evidence for saturability?
Nick
1. Tham LS, Wang L, Soo RA, Lee SC, Lee HS, Yong WP, et al. A
pharmacodynamic model for the time course of tumor shrinkage by
gemcitabine + carboplatin in non-small cell lung cancer patients. Clin
Cancer Res. 2008a;14(13):4213-8.
2. Tham LS, Wang LZ, Soo RA, Lee HS, Lee SC, Goh BC, et al. Does
saturable formation of gemcitabine triphosphate occur in patients?
Cancer Chemother Pharmacol. 2008b. Epub February.
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.at.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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Frederik,
Thanks for pointing out the claims of the gemcitabine study (Grimison
et al 2007) which I find unconvincing. If you read past the quote you
made from the abstract you will find there is very little reported
about the PKPD relationship (%D is the percent drop in white cell
count after one week compared to baseline):
"Relationship between pharmacokinetics and pharmacodynamics.
A relationship was found between the GEM-TP AUC and %D, with
best fit using a sigmoid Emax model (Fig 4A) and r2 =0.3739. When
GEM-TPsamples were divided at the median (81.0 mol/million cells
x h) into high and low groups, the high GEM-TP group was associated
with greater%D(P<0.01 Fig 4B).
No correlation was found between %D and the GEM Cmax, nor
between%Dand the GEM AUC."
Note that the correlation coefficient for GEM-TP is only 0.37 and was
not reported for GEM parent. There is no attempt to discuss the power
of the analysis to detect any relationship, not even a confidence
interval, so it is impossible to conclude if there is really any
difference between GEM-TP and GEM parent exposure-response
relationships. A glance at Figure 4A gives me no confidence in their
claimed sigmoid relationship between GEM-TP and the drop in white cell
count (although I have no doubt it exists). It appears to predict a
25% drop in white cells even when GEM-TP approaches zero and an almost
on/off switch like lowering of white cells over a very narrow range of
GEM-TP concs. A comparable plot for GEM parent is not shown and so I
think any statement about which is more predictive is just speculation.
Remember that "the absence of evidence is not evidence for absence" !
The deficiencies of the analysis and the confounded conclusions from
this study are discussed in an accompanying editorial (Gandhi 2007)
in the same issue: "Unfortunately, even though data were available,
the authors did not perform paired analyses for each patient but,
rather, expressed and compared data as the mean and standard deviation
for each group. With a reasonable number of patients in this optimally
designed clinical trial, the answers to the gemcitabine dose rate
questions were expected to be achieved. However, an unexpected result
was noticed that confounded the conclusion. " (the editorial goes on
to speculate about auto-induction of deoxycytidine kinase).
Nick
Grimison P, Galettis P, Manners S, Jelinek M, Metharom E, de Souza PL,
et al. Randomized Crossover Study Evaluating the Effect of Gemcitabine
Infusion Dose Rate: Evidence of Auto-Induction of Gemcitabine
Accumulation. J Clin Oncol. 2007;25(36):5704-9.
Gandhi V. Questions About Gemcitabine Dose Rate: Answered or
Unanswered? J Clin Oncol. 2007;25(36):5691-4.
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.-a-.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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The following message was posted to: PharmPK
Hi Nick,
Thank you; I hadn't seen the editorial (easy to miss if you go straight
to the article).
Although the study analysis may be unconvincing (inconclusive) the
authors do state:
"However, the relatively low correlation coefficient suggests that
intracellular accumulation does not determine the majority of the
variation
in drug response (for neutrophils)."
They also mention the following, which is pertinent to one of Dr. Wolf's
comments:
"This work used leukocytes as an accessible and relevant surrogate
for hematologic toxicity. The relevance of leukocyte uptake for
predicting
changes in tumor uptake also needs to be explored."
Finally, I think this is a "watch this space" one; FWIW the study was
supported by a research grant from Eli Lilly.
Best regards,
Frederik Pruijn
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Frederik,
Trying to learn how the world works by divination of correlation
coefficients is as about as useful as reading tea leaves.
Basically the design and analysis of that study from the viewpoint of
trying to learn what information can be gleaned from plasma vs
intracellular white cell concentration was of little value.
If you read the papers I originally cited from Tham et al. you will
see that the difficulties of relying on white cells to predict tumour
concs and kinetics are acknowledged.
I look forward one day to seeing that more reviewers of 'leading'
journals such as JCO will recognize the advances made in understanding
pharmacology from a quantitative perspective and not allow poorly
thought out stuff to reach the light of day. Thankfully, at least one
clever paper (Friberg et al. 2002) that Grimison et al. could have
usefully digested got past the system into JCO.
Nick
Friberg LE, Henningsson A, Maas H, Nguyen L, Karlsson MO. Model of
Chemotherapy-Induced Myelosuppression With Parameter Consistency
Across Drugs. J Clin Oncol. 2002;20:4713-21.
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.-at-.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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The following message was posted to: PharmPK
Hi all,
One important issue to use intracellular triphosphate t1/2 of
elimination is to support the once-a-day dosing for several
antiretroviral nucleosides such as lamivudine and tenofovir which
exhibit long t1/2. With respect to short plasma t1/2, twice-a-day
dosing would be chosen.
Regarding relation-ship between either plasma or intracellular (white
blood cells) levels and efficacy, it is well known that plasma
concentrations of antiretroviral nucleosides (also called Nucleosides
Reverse Transcriptase Inhibitors) are not related with patient
outcome. However, it has not been formally stated that, in a large
population of patients, intracellular concentration could be more
related. Many studies have shown the interest of intracellular data,
but often with few patients. Studying these kind of relation-ships is
quite complex since many factors are involved, such as :
1- these drugs are in competition with endogenous nucleotides
triphosphate and the ratio between the endogenous nucleotide
triphosphate versus the drug triphosphate could be more appropriate,
2- These drug are given as part as multitherapies and the impact of
one drug is therefore difficult to be separately evaluated,
3- the genotype of the virus (drug target) is of main importance in
the antiviral therapy, and for some authors, is even more relevant
than the drugs concentrations.
Several of these considerations can be extrapolated to anticancer
nucleosides.
Henri BENECH
CEA
Service de Pharmacologie et d'Immunoanalyse
Institut de Biologie et de Technologies de Saclay
FRANCE
email: henri.benech.at.cea.fr
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Henri,
Would you be kind enough to give references to the original scientific
literature to support your assertions?
Assertion #1
> One important issue to use intracellular triphosphate t1/2 of
> elimination is to support the once-a-day dosing for several
> antiretroviral nucleosides such as lamivudine and tenofovir which
> exhibit long t1/2. With respect to short plasma t1/2, twice-a-day
> dosing would be chosen.
I asked earlier in this thread for literature support for this statement
about using so called intracellular half-lives to predict dosing
intervals and the only one offered in fact had no evidence at all.
Assertion #2
> ...it is well known that plasma concentrations of antiretroviral
> nucleosides (also called Nucleosides Reverse Transcriptase
Inhibitors)
> are not related with patient outcome.
I find this rather hard to accept because I cannot imagine how NRTIs can
influence outcome without causing changes in plasma concentration. Any
study that shows a benefit of NRTIs must therefore have a relationship
between plasma conc and outcome. I am quite happy to accept that the
plasma conc is just measuring a prodrug for the active species but that
doesn't mean the plasma conc cannot be related to the outcome.
Unfortunately most of those working in this area dont seem to want to
apply modern methods of PKPD analysis to understand the relationships.
There is more to it than measuring some concentrations and computing a
correlation coefficient. Are you able to provide evidence based on an
appropriately powered and analysed trial that plasma concs of NRTIs are
not related to patient outcome?
Assertion #3
> However, it has not been formally stated that, in a large population
> of patients, intracellular concentration could be more related. Many
> studies have shown the interest of intracellular data, but often with
> few patients.
I cited the study of Tham et al (2008) which demonstrated in 56 patients
that tumour response is predictable from both plasma gemcitabine and its
intracellular triphosphate metabolite. However, the intracellular TP was
no better at predicting the response than either plasma concentration or
dose. While this is a different disease and different drug class from
the NRTIs if you read the paper you will find a discussion of the
difficulties of using intracellular triphosphate measurements to predict
response.
Nick
Tham LS, Wang L, Soo RA, Lee SC, Lee HS, Yong WP, et al. A
pharmacodynamic model for the time course of tumor shrinkage by
gemcitabine + carboplatin in non-small cell lung cancer patients. Clin
Cancer Res. 2008;14(13):4213-8.
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.-a-.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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The following message was posted to: PharmPK
The paper by Stein DS and Moore KHP on Phosphorylation of Nucleoside
Analogs Antiretrovirals: A Review for Clinicians in Pharmacotherapy
2001; 21(1):11-34 may provide some of the answers you seek. The
phosphorylation of NRTIs is the rate-limiting step to generating the
pharmacologically active moiety (NRTI-triphosphate or -diphosphate in
the case of tenofovir) so plasma concentrations in general do not
correlate to intracellular concentrations. Further complicating
matters, different cell types will phosphorylate at different rates.
The PKPD implication is that HIV-infected macrophages may respond
differently than HIV-infected lymphocytes and neither is likely
predicted by plasma NRTI concentrations. Also, the phosphorylated
moiety becomes trapped within the cell thus the typically longer
intracellular half-life compared to plasma. I do agree that the
technical difficulty in assaying intracellular concentrations limits
what we know about the PKPD of NRTIs and unfortunately only a few
academic labs around the world devote their time and efforts to this.
Regards,
Thomas Kakuda, Pharm.D.
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The following message was posted to: PharmPK
Dear Nick,
1) Your Assertion #1: I asked earlier in this thread for literature
support for this statement about using so called intracellular half-
lives to predict dosing intervals and the only one offered in fact had
no evidence at all.
< You can refer to the following paper and especially in Fig 1 and Fig
2 showing the t1/2 in plasma and in the cells:
Antimicrob Agents Chemother. 2004 Jan;48(1):176-82.
Equivalent steady-state pharmacokinetics of lamivudine in plasma and
lamivudine triphosphate within cells following administration of
lamivudine at 300 milligrams once daily and 150 milligrams twice
daily.Yuen GJ, Lou Y, Bumgarner NF, Bishop JP, Smith GA, Otto VR,
Hoelscher DD.
Thus, lamivudine is now given once-daily whereas zidovudine is still
given twice-daily, the latter exhibiting a too short intracellular t1/2.
Antimicrob Agents Chemother. 2007 Oct;51(10):3516-22. Epub 2007 Jul
30. Links
Intracellular pharmacokinetics of once versus twice daily zidovudine
and lamivudine in adolescents.Flynn PM, Rodman J, Lindsey JC, Robbins
B, Capparelli E, Knapp KM, Rodriguez JF, McNamara J, Serchuck L,
Heckman B, Martinez J; PACTG P1012 Team.
2) Your Assertion #2: Are you able to provide evidence based on an
appropriately powered and analysed trial that plasma concs of NRTIs
are not related to patient outcome?
< You can refer to this paper indicating that plasma AUC of zidovudine
was not correlated with markers of therapeutic response in HIV-
infected patients.
AIDS. 1994 Jun;8(6):763-9.
Correlates of zidovudine phosphorylation with markers of HIV disease
progression and drug toxicity.Stretcher BN, Pesce AJ, Frame PT,
Greenberg KA, Stein DS.
3) Assertion #3:
< You can refer to the previous paper of Stretcher et al. as well as
the following two others indicating in few patients a relationship
between intracellular triphosphate and virologic or immunologic
response.
- AIDS. 2000 Sep 29;14(14):2137-44. Zidovudine triphosphate and
lamivudine triphosphate concentration-response relationships in HIV-
infected persons.Fletcher CV, Kawle SP, Kakuda TN, Anderson PL, Weller
D, Bushman LR, Brundage RC, Remmel RP.
- Antivir Ther. 2007;12(6):981-6. Intracellular nucleoside
triphosphate concentrations in HIV-infected patients on dual
nucleoside reverse transcriptase inhibitor therapy.Moore JD, Acosta
EP, Johnson VA, Bassett R, Eron JJ, Fischl MA, Long MC, Kuritzkes DR,
Sommadossi JP.
The discussion is still open...
Henri BENECH
CEA
Service de Pharmacologie et d'Immunoanalyse
Institut de Biologie et de Technologies de Saclay
FRANCE
email: henri.benech.-at-.cea.fr
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One of the issues in this discussion is: the use of plasma or
intracellular concentration as a predictor of virologic success. Most
of the evidence that claims plasma concentrations are NOT predictors
of virologic success was derived from the use of (binary) endpoints at
a single time point (for example, week 24 or 48 etc). These do not
provide any information on time course for sure. In modeling viral
dynamics, I am not aware of a lot of efforts being put into linking
plasma concentration to intracellular concentration and then to the
change in viral load. It is definitely ideal but very hard to do and
especially with the kind of PKPD data available from clinical trials.
Here discussion on intracellular half life and IC50, probably, got
mixed up in the thread. Plasma NRTI concentrations are still
surrogates of antiviral response for NRTIs. For example, matching
exposures in pediatrics and adults is used as a basis for pediatric
dosing recommendations for NRTIs. And no one can argue that NRTI
plasma concentrations are irrelvant to viral response and
intracellular concentrations, if available reliably, will be better
predictors- because it is closer to mechanism of action.
Further, I am not aware of any NRTI for which the dosing interval was
chosen primarily based on intracellular half life. The dosing
intervals were explicitly studied (for most part) in clinical trials.
For example, zidovudine (TID vs BID in adults), lamivudine (BID vs QD)
have gone through such transformations. The intracellular half life,
however, has been used a basis fror hypothesis generation. It worked
for zidovudine or lamivudine but it did not work that well for DDI.
Currently, BID is a preferred strategy than QD (see DDI label). So
understanding of time course should work better but intracellular half
life has been used as a guiding principle to just start thinking about
it.
Pravin
Pharmacometrics reviewer
OCP/OTS/FDA
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Pravin,
Thanks for your helpful comments. I presume these are not official FDA
statements :-)
You wrote:
> Here discussion on intracellular half life and IC50, probably, got
> mixed up in the thread.
Actually the mixup is in the FDA antiviral guidance. I pointed this out
when I first contributed to the thread...
>
> 'For nucleoside analog drugs, the FDA guidance on Antiviral Product
> Development recommends "that sponsors determine the intracellular
> half- life (t1/2) of the triphosphate form of the active drug moiety
> in stationary and dividing cells from the target tissue." '
>
> This appears to be a quote from
> http://www.fda.gov/Cder/guidance/7070fnl.htm.
>
> The context is a discussion of the EC50. The recommendation about
> determining the intracellular half-life seems to be a non-sequitur
> (i.e. does not follow from the preceding material).
Nick
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.-a-.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
Back to the Top
Henri,
Thanks very much for responding to my request and providing references
to the literature for discussion.
> 1) Your Assertion #1: I asked earlier in this thread for literature
support for this statement about using so called intracellular half-
lives to predict dosing intervals and the only one offered in fact had
no evidence at all.
The paper by Yuen 2004 is a PK study. It does not provide any evidence
about the effect of dosing intervals on outcome so I dont see that it
provides support for using intracellular half-lives as a way to
predict the dosing interval.
> 2) Your Assertion #2: Are you able to provide evidence based on an
appropriately powered and analysed trial that plasma concs of NRTIs
are not related to patient outcome?
The paper by Stretcher 2004 is based on 21 patients. There was no
correlation betwen zidovudine triphosphate AUC and absolute change in
any biomarker. A statistical correlation was only seen with % change
from baseline in 3 out of 24 biomarkers. This is rather a weak
finding. It also provides no explanation for the difference between
AUC of plasma vs intracellular ZDV TP. This seems like a statistical
quirk of an underpowered study. It is certainly not powered to
demonstrate that plasma concs are not related to patient outcome.
The paper of Fletcher 2000 is only 8 patients and with rather sparse
sampling so that AUC of plasma concs was not available.
I haven't been able to find an online copy of Moore et al. 2004 but
the abstract does not refer to plasma concs and biomarker changes.
So it seems that the data that plasma concs are not correlated with
biomarkers and with outcome is non-existent. There is no evidence that
relying in intracellular half-lives is useful for designing dosage
regimens.
The discussion is still open?
Nick
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.aaa.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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Hi Nick
Thanks for pointing it out. It has been a long thread.
You wrote:
> Actually the mixup is in the FDA antiviral guidance. I pointed this
out when
> I first contributed to the thread
I am reading it differently than you are. http://www.fda.gov/Cder/guidance/7070fnl.htm
In the section you're referring to, it speaks about several
measurements that could be conducted- for example, antiviral activity
(IC50, genotype data), cytoxicity etc. Then it goes on to add a
comment on intracellular half life for nukes. I think they can be
taken exclusive comments. Could it be stated more clear? Will help
probably.
But if you read further, section B3 on inhibitory quotient talks about
relating intracellular or plasma concentrations to determine dose-
response. It does not give any reference to intracellular half life.
Yes, these are personal comments and do not reflect official position
of the government or FDA.
Thanks
Pravin
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The following message was posted to: PharmPK
Knowing the half-life for intracellular NRTIs triphosphate can also be
useful to provide information when the treatment is modified or stopped.
For example, intracellular tenofovir diphosphate remains detectable
within the circulating cells several weeks after stopping the
treatment (whereas plasma tenofovir is no longer present after a few
days)....Knowing that the active metabolite is still present in the
body, may be at sub-therapeutic levels, during several weeks after
treatment interruption should be relevant, either for the
pharmaceutical company or the clinician.
These kind of therapeutic are complex and it would be a pity not to
know the behaviour of the active metabolite into the cells. In vitro
experiment using radiolabeled drug and/or LC-MS/MS can provide
information quite easily. For in vivo, the challenge is not the same.
Regards
Henri
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Henri,
> Knowing that the active metabolite is still present in the body, may
> be at sub-therapeutic levels, during several weeks after treatment
> interruption should be relevant, either for the pharmaceutical
company
> or the clinician.
If I had HIV I would not want my clinician to wait for several weeks
after stopping treatment because of some observational studies of the
presumed active metabolite. This kind of treatment interruption is
associated with a high risk of developing resistant virus.
HIV scientists need to get realistic about what is convenient to measure
in a test tube and what is predictive of clinical outcome. There are
many more steps in the chain between dose and clinical outcome after
generation of a triphosphate metabolite. I am pretty sure it is the time
course of drug exposure in the plasma that is the driver of clinical
outcome. At present this is the only thing that can be controlled e.g.
dose size, dosing frequency, slow release formulations. Until it is
possible to change intracellular concentrations independently of plasma
concs then these intracellular studies are just observational
curiosities.
Nick
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.at.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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Pravin,
This thread started because of the FDA 'guidance' referring to
intracellular half-life. The document
http://www.fda.gov/Cder/guidance/7070fnl.htm
does indeed mention a range of other things but there is no
information on what use should be made of the intracellular half-life.
IMHO it can only make sense in the context of a comprehensive PKPD
model that accounts for the full time course of plasma and perhaps
intracellular concs, the shape of the conc-response relationship for
viral kill and most importantly the relationship between viral kill
and clinical outcome. The intracellular half-life might be part of the
overall picture but so far the contributions to this thread have not
provided scientific evidence for its role.
Nick
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
n.holford.-at-.auckland.ac.nz
http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford
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The following message was posted to: PharmPK
Nick,
You wrote: I am pretty sure it is the time course of drug exposure in
the plasma that is the driver of clinical outcome. At present this is
the only thing that can be controlled e.g. dose size, dosing
frequency, slow release formulations. At present this is the only
thing that can be controlled e.g. dose size, dosing frequency, slow
release formulations. Until it is possible to change intracellular
concentrations independently of plasma concs then these intracellular
studies are just observational curiosities.measuring the drug, and it would not be the case for plasma prodrug
and intracellular active metabolite?
I do think that the reticences come from 1) the difficulty of
measuring these intracellular triphosphates in patients easily and
with the highest confidence, and 2) from the fact that in multitherapy
the impact of only one drug is very difficult to be evaluated without
measuring all the other drugs as well as knowing their respective
power on the virus present in each patient.
You ask for a study indicating that change in intracellular
nucleotides may be related to efficacy and/or toxicity without any
change in the plasma compartment?
Please refer to the following papers:
1) Intracellular interaction between didanosine and hydroxyurea:
http://www.ncbi.nlm.nih.gov/pubmed/18184078?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
AIDS Res Hum Retroviruses. 2007 Nov;23(11):1360-5.Effect of
hydroxyurea and dideoxyinosine on intracellular 3'-deoxyadenosine-5'-
triphosphate concentrations in HIV-infected patients.Bakshi RP, Hamzeh
F, Frank I, Eron JJ Jr, Bosch RJ, Rosenkranz SL, Cramer YS, Ussery M,
Flexner C.Nevertheless, if the discussion is no longer open, the works
are going on.
In addition the intracellular interaction between zidovudine and
stavudine, despite no modification of the plasma levels is well known :
http://www.ncbi.nlm.nih.gov/pubmed/10882616?ordinalpos=11&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
J Infect Dis. 2000 Jul;182(1):321-5. Epub 2000 Jul 6. Links
In vivo antagonism with zidovudine plus stavudine combination
therapy.Havlir DV, Tierney C, Friedland GH, Pollard RB, Smeaton L,
Sommadossi JP, Fox L, Kessler H, Fife KH, Richman DD.
If the discussion is not still open, fortunately the experiments still
go on...
Regards
Henri BENECH
CEA
Service de Pharmacologie et d'Immunoanalyse
Institut de Biologie et de Technologies de Saclay
Bat. 136, point courrier N*18
91191 Gif-Sur-Yvette Cedex, France
email: henri.benech.at.cea.fr
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The following message was posted to: PharmPK
Nick,
You wrote:" There is no evidence that relying in intracellular half-
lives is useful for designing dosage regimens."
If you refer to this recent review:
http://www.ncbi.nlm.nih.gov/pubmed/18370438?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
Drugs. 2008;68(5):567-78. Efficacy and safety of once-daily regimens
in the treatment of HIV infection.Molina JM.
You will see that the NRTIs used once daily (tenofovir, didanosine,
lamivudine and emtricitabine) are the ones exhibiting the longer t1/2
of elimination: higher than 24 h). On the contrary, for zidovudine and
stavudine, still used twice daily, their t1/2 are lower than 12 hour).
This seems to be quite informative.
We all know that determining an appropriate regimen should be proved
in a large scale of patients. But in the fight again HIV, clinicians
asked every days for more convenient treatment. Intracellular t1/2 are
an useful help (starting point?) for successfully testing once-daily
regimen, despite very short plasma t1/2.
Henri BENECH
CEA
Service de Pharmacologie et d'Immunoanalyse
Institut de Biologie et de Technologies de Saclay
Bat. 136, point courrier N*18
91191 Gif-Sur-Yvette Cedex, France
email: henri.benech.at.cea.fr
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The following message was posted to: PharmPK
Having just caught up with this, I wonder if any of the papers that
Adrian
Harris published on cytosine arabinoside (cytarabine) phosphorylation
in human
leukaemic myeloblasts nearly 30 years ago contributes to the
discussion. The
references are:
Br J Clin Pharmacol 1979;8(3):219-27.
Br J Haematol 1980;45(3):371-9.
Cancer Chemother Pharmacol 1981;5(3):185-92.
Clin Sci (Lond) 1981;60(2):191-8.
Cancer Chemother Pharmacol 1982;9(1):30-5.
--
J K Aronson, University Department of Primary Health Care,
Rosemary Rue Building, Old Road Campus, Roosevelt Drive, Headington,
Oxford OX3 7LF
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