2. Learning Objectives
Identify primary drug interaction concepts
Describe types and mechanisms of
interactions
Identify drug interactions commonly
encountered with antiretroviral drugs
Describe how to manage known
interactions
3. Definition:
The pharmacological result, either desirable
or undesirable, of drugs interacting with
themselves or with other endogenous
chemical agents, components of the diet, or
with chemicals used in or resulting from
diagnostic tests.
4. Case Study: Lake
Lake, a 50 year-old male who has been HIV+ for 5
years and is stable on therapy, presents to the
clinic to get more medication to treat his thrush
He has been taking his brother’s medication, which
seemed to help at first and then stopped working.
He would like to get some more to clear the white
plaques on his tongue
6. Case Study: Lake (3)
His current ARV regimen is:
Nevirapine 200 mg bid
Zidovudine 300 mg bid
Lamivudine 150 mg bid
He has one pill of his brother’s medication left.
The physician brings it to the pharmacy to
determine what medication it is. The tablet is
identified as ketoconazole 200 mg
7. Case Study: Lake (4)
Is this an appropriate medication to use with his
current ARV regimen?
What are some counseling points for this patient?
8. Beware
A drug interaction can occur whenever a:
New medication is started
Medication is discontinued
Dose is changed
Drug is changed
Remember:
Inducing interactions
Gradual onset/offset
Inhibiting interactions
Quick onset/offset
9. Mechanisms for Drug Interactions
Pharmacokinetic Interactions
Altered drug absorption and tissue distribution
Chelation, pH, efflux proteins or drug transporters)
Altered drug metabolism
Induction/inhibition
Reduced renal excretion
Altered intracellular activation
Impairment of phosphorylation (D4T, ZDV)
The outcome of these interactions could be
additive/synergistic, antagonistic/opposing or
potentiation
10. Mechanisms for Drug Interactions (2)
Pharmacodynamic interactions
Additive or synergistic interactions
Antagonistic or opposing interactions
11. Recognize that metabolism can occur in the intestines,
liver or blood
Route of orally administered drugs:
Absorbed in the gastrointestinal tract
Then pass through the portal venous system to the
liver where they are exposed to first pass effect,
which may limit systemic circulation
Once in the systemic circulation, drugs interact with
receptors in target tissues
First Pass Effect
12. Cytochrome P450 (CYP450)
Substrate
Medication depends on enzymatic pathway(s) for
metabolism
Object drug which is affected by inducer or inhibitor
Inducer
Speeds up metabolism
Decreases substrate level (lack of efficacy is concern)
Gradual onset/offset
Inhibitor
Slows metabolism
Increases substrate level (toxicity is concern)
Quick onset/offset
13. Cytochrome P450 Enzymes
Outcome of
Drug
Interaction
Variability
Patient Factors Drug Factors
•Genetics
•Diseases
•Diet/Nutrition
•Environment
•Smoking
•Alcohol
•Dose
•Duration
•Dosing Times
•Sequence
•Route
•Dosage Form
17. Interactions among HIV drugs
itself: NRTIs
Most important are 2 types of interactions:
• Do not combine 2 NRTIs that require same
enzymes for intracellular phosphorylation:
– d4T + AZT
– ddC, FTC, 3TC
• Do not combine TDF with ddI
– Increased ddI toxicity
– Loss of immunological response
18. NNRTIs are inducers of CYP3A
• PIs are substrates of CYP3A
• When combining NNRTIs with PIs, usually
the dose of the PI is increased, for
example:
– LPV/r 533/133 (4 caps) BID + EFV, or
– LPV/r 600/150 (3 tabs) BID + EFV
Interactions among HIV drugs
itself: NRTIs…
19. Red Flags for Potential Interactions
PIs or NNRTIs and
Ergot alkaloids
Azole antifungals
Antihistamines
Anticonvulsants
Anti-tuberculars (rifamycins)
Warfarin
Benzodiazepines
Cardiac medicine
Amiodarone, quinidine
Oral contraceptives
Containing estradiol
Macrolide antibiotics
Methadone
20. PI/ NNRTI/ Antidepressant
Drug Interactions
As above
Levels of
sertraline may
be increased.
ARV levels
not likely to
change.
ritonavir,
lopinavir/r, all
other Pis,
efavirenz
Sertraline
As above
Levels of both
fluoxetine and
ARVs may be
increased
ritonavir,
lopinavir/r, all
other PIs,
efavirenz
Fluoxetine
Start with lower dose
(50%) of amitriptyline,
adjust dose when
addIng ritonavir.
Monitor for side
effects
Levels of
amitriptyline may
be increased
ritonavir,
lopinavir/r,
amprenavir,
Amitriptyline
Management
Effects
Potential for
Interaction
Antidepressant
22. NNRTIs: Do NOT Co-administer
Ergot derivatives (ergotamine)
Benzodiazepine: midazolam, triazolam
Rifampicin (Nevirapine) – unless there is NO
alternative
Terfenadine (Efavirenz)
Herbal – St. Johns wort
23. Food-Drug Interactions
A food-drug interaction can occur when the food
you eat affects the ingredients in a medication you
are taking, preventing the medicine from working
the way it should. Food-drug interactions can
happen with both prescription and over-the-counter
medications, including antacids, vitamins, and iron
pills.
24. Food-Drug Interactions…
Points to note
-Advise patients to take medication with a full glass of
water.
-Not stir medication into food or take capsules apart (unless
directed by your physician).
-Do not take vitamin pills at the same time you take
medication (i.e, take medication 1 hour after taking
vitamins).
-Not mix medication into hot drinks, because the heat from
the drink may destroy the effectiveness of the drug.
-Never take medication with alcoholic drinks.
-Ask the patient about all medications they are taking, both
prescription and non-prescription.
25. Antiretroviral/Food Interactions
Take with food:
Lopinavir (capsules or
solution): ↑ 50-130%
Avoid food:
ddI: 47% ↓ with meal
Efavirenz: ↑ 79% high fat meal
increases toxicity
Rifampin: food may ↑ levels
Isoniazid
28. Case Study: Endalk
Endalk is 45 year-old HIV+ male presenting for
routine follow-up. He has been on HAART for two
years. CD4 count: 480 cells/mm3 HIV RNA < 50
copies/mL.
He comes into the pharmacy after seeing a
physician for his migraines. He is glad to try a new
medication as his headaches have been a problem
for years. He is so distraught about them that he
has begun to take an herbal product to help with
his mood
29. Case Study: Endalk (2)
His current medication regimen, which is:
Nevirapine 200 mg bid
Lamivudine 150mg bid
Zidovudine 300 mg bid
An herbal medicine when he feels “down”
New medications prescribed today: Ergotamine
+ caffeine
30. Case Study: Endalk (3)
Which of the following combinations
represents a potential drug-drug
interaction?
A. Nevirapine and herbal medicine
B. Zidovudine and ergotamine
C. Ergotamine and nevirapine
D. Caffeine and zidovudine
31. Case Study II: Sara
Sara is a 41 year-old female with esophageal
candida and has just completed a 10 day course
of fluconazole. She has lost weight because
symptoms of thrush made it difficult to swallow.
She weighs 62 kg. She is to begin ARV therapy
today.
32. Case Study: Sara (2)
She presents with the following
prescription:
Zidovudine 300 mg bid
Stavudine 40 mg bid
Nevirapine 200 mg once daily for the first 2
weeks, then increase to 200 mg bid
Cotrimoxazole DS, 1 tablet daily
2. Is this an appropriate regimen for her? Can
you identify any possible drug interactions
33. Case Study: Lake
Lake, a 50 year-old male who has been HIV+ for 5
years and is stable on therapy, presents to the
clinic to get more medication to treat his thrush
He has been taking his brother’s medication, which
seemed to help at first and then stopped working.
He would like to get some more to clear the white
plaques on his tongue
35. Case Study: Lake (3)
His current ARV regimen is:
Nevirapine 200 mg bid
Zidovudine 300 mg bid
Lamivudine 150 mg bid
He has one pill of his brother’s medication left.
The physician brings it to the pharmacy to
determine what medication it is. The tablet is
identified as ketoconazole 200 mg
36. Case Study: Lake (4)
Is this an appropriate medication to use with his
current ARV regimen?
What are some counseling points for this patient?