2. Items
• Mode of Action.
• Uses and Doses.
• Side Effects.
• Drug Interactions.
• Units Conversion.
• Key Parameters.
• Bioavailability.
• Volume of Distribution.
2
3. Items
• T ½.
• Clearance.
• Goal of Monitoring.
• Therapeutic Plasma
Concentration.
• Sampling and Assay.
• Leucovorin.
• Conclusion.
3
4. Mode of Action
• MTX is a folic acid anti-metabolite.
• MTX acts via inhibition of DHFR.
• The affinity of DHFR to
methotrexate is far greater than
its affinity to folic acid or
dihydrofolic acid.
4
8. Uses & doses
• The uses of LDMTX include:
• Rheumatoid arthritis
• Psoriasis
• Psoriatic arthritis
• Ectopic pregnancy.
• The LDMTX range from
( 7.5mg to 25mg )
weekly administered as a single dose or split up into
3 doses every 12 hours.
8
9. Uses & doses
• HDMTX is used to treat certain
types of cancer such as:
– Leukemia
– Non-Hodgkin's lymphoma
– Breast cancer
– Head and neck cancers
– Stomach cancer
– Bladder cancer
– Choriocarcinoma.
9
10. Uses & doses
• The recommended dosage of
methotrexate for treating cancer
can vary significantly.
• It is based on :
– The type of cancer,
– The patient’s weight,
– How well the patient can tolerate the
drug.
10
11. • For some types of cancer,
methotrexate is given:
once a week
• for other types, it is given:
cyclically,
(such as a five-day course of
treatment with a week-long -or
longer- breaks between treatments.)
11
21. • To convert methotrexate
concentrations in units of
mgL molar
Conversion Formula as below
• MTX 𝑐𝑜𝑛𝑐. 𝑖𝑛 10ˉ⁶ 𝑚𝑜𝑙𝑎𝑟 =
𝑴𝑻𝑿 𝒄𝒐𝒏𝒄. 𝒊𝒏 𝒎𝒈𝑳
𝟎. 𝟒𝟓𝟒
Mwt of MTX = 454 g/mole.
Units Conversion
(cont.)
21
22. Units Conversion
(cont.)
• To convert methotrexate
concentrations from
10ˉ⁶ molar mgL
APPLY the following formula
• 𝑴𝑻𝑿 𝒄𝒐𝒏𝒄. 𝒊𝒏 𝒎𝒈𝑳 =
(MTX conc. In 10⁻⁶ molar) x (0.454 )
22
23. Half-Life T ½
αT ½ 3 hrs
βT ½ 10 hrs
Key Parameters
Therapeutic Plasma Concentration Variable
23
Toxic Plasma Concentration
Plasma >1 x 10-7 molar for > 48 hrs
> 1 x 10-6 molar at > 48 hrs ↑ leucovorin
rescue doses.
CNS continuous CNS methotrexate conc. > 10-8
molar
Bioavailability
Dose <30 mg/m2 100%
Dose >30 mg/m2 Variable
Volume of Distribution
Vi (Initial) 0.2 L/Kg
V (AUC) 0.7 L/Kg
Clearance [1.6] [CLcr]
25. Bioavailability
• Doses < 30 mg:
– The drug is completely absorbed
– Peak conc. occurs 1-2 hours after administration.
– Doses < 30 mg are available either orally or
parenterally.
25
26. Bioavailability
• Doses > 30 mg:
– Bioavailability declines due to
saturable absorption.
– Doses > 30 mg are only available parenterally.
26
27. Volume Of Distribution (Vd)
• MTX displays at least bi-exponential
elimination curve indicating:
– Initial Vd 0.2 L/Kg,
– Second Vd 0.2 – 1 L/Kg.
• This makes calculations rather
complicated.
• However, when LD calculations are
required:
– Vd from 0.2 – 0.5 L/Kg is usually
employed.
27
28. • MTX Vd also appears to INCREASE
at higher plasma concentrations.
• This reflects an active transport
mechanism which becomes
saturated at higher concentrations
reverts to passive intracellular
diffusion.
28
Volume Of Distribution (Vd)
29. Third Spacing
• The presence of third-space fluids
can influence Vd;
• Examples of third space fluids:
– Ascites,
– Edema,
– Pleural effusion.
29
30. Half Life (T ½ )
• The apparent half-lives for MTX are
determined by both :
– A changing volume of distribution,
– A changing clearance.
• The relationship between MTX’s volume
of distribution and clearance is
complex
– Because of the capacity limited
intracellular transport and capacity limited
renal clearance.
30
31. • A simple two- compartment model
with:
– An initial α T ½ of 3 hours
– A terminal β T ½ of 10 hours
Appears to represent the elimination
phase reasonably well.
• The terminal (β) T ½ doesn’t become
apparent until plasma concentrations
decline into the range of (0.5x10⁻⁶) molar.
31
Half Life (T ½ )
33. • Significant MTX is eliminated during
the α phase, this means that:
– A very large percentage of the total
methotrexate dose may be eliminated
during the α phase.
• Nevertheless the β phase is also
important, because:
Retention of even a very small amount of
the administered dose can be toxic to the
patient.
33
Half Life (T ½ )
34. Pleural Effusions
• Pleural effusions do not substantially
increase Vd.
• But high drug concentrations
34
Accumulate in the fluids,
Delay equilibrium with plasma,
Delay clearance,
Toxicity.
35. • Prolongation of the terminal
elimination phase can:
• Extend the time needed to
achieve plasma conc. Of
0.1 x 10ˉ⁶ molar.
• Patient will require additional
doses of rescue factor.
35
Pleural Effusions
36. T ½ =
0.693
𝐾
.. (hr)
K =
0.693
𝑡1/2
.. (hr -1 )
T =
ln(
𝐶1
𝐶2
)
𝐾
… (hr)
36
Half Life (T ½ )
37. • The vast majority of methotrexate is
eliminated by: the renal route.
• Methotrexate clearance ranges from
(1 – 2) times the creatinine clearance.
(Clinical calculations use a factor of 1.6).
37
Clearance (Cl)
• This variability in renal clearance is due to :
the renal methotrexate clearance by active transport
mechanisms that may become saturable.
38. The renal clearance of methotrexate is
influenced by a number of compounds
such as:
»Probenecid.
»Salicylate.
»Sulfisoxazole.
• These compounds diminish the
renal transport of methotrexate.
38
Clearance (Cl)
39. • A relatively small percentage of
methotrexate is metabolized.
• Nevertheless, significant amounts of
methotrexate metabolites can be
found in the urine when large doses are
administered.
• The most extensively studied metabolite is:
7-hydroxy-methotrexate.
39
Clearance (Cl)
42. Therapeutic Cp
• Most therapeutic regimens are
designed to achieve plasma
methotrexate concentrations above
1x 10ˉ⁷ molar for less than 48 hours.
• Concentrations of methotrexate that
have been associated with treatment
of various neoplasms range from :
10ˉ⁶ molar up to 10⁻³ or 10⁻² molar.
42
43. Therapeutic Cp
• These high methotrexate levels are not
usually associated with serious
methotrexate toxicity as long as:
1. Adequate hydration and renal function
are maintained.
2. MTX concentration falls below 1x10⁻⁷
molar within 48 hours following the
initiation of therapy or the
discontinuation of leucovorin rescue.
43
44. Toxic Cp
• Plasma concentrations
exceeding 1x10⁻⁷ molar for 48
hours or more,
are associated with methotrexate toxicity.
• Most common toxic effects include:
1. Myelosuppression.
2. Oral and gastrointestinal mucositis.
3. Acute hepatic dysfunction.
44
45. • To evaluate efficacy:
– Prolonged infusions adjustments in inf. rate.
– Repeated doses adjustments in future doses.
45
Goal of MTX Monitoring
• To ensure that all patients are receiving:
adequate doses of leucovorin to prevent
methotrexate toxicity.
Esp. Patients on HDMTX (> 500mg/m²).
• To detect unusual MTX disposition characteristics that
could result in serious toxicity.
46. Sampling
When Do We Sample?
• Sampling before the critical 48 hours:
– Indicate if the elimination of MTX is normal.
– NEVER extrapolate the data to more than 2 T½.
• Sampling 24 – 48 hours following initiation
of therapy:
– Determine the quantity/duration of required
leucovorin.
• Some protocols sample
* during the infusion,
* at 48 hours &
* every 24 hours until the patient is rescued.
46
47. Assay
• There are various available methods.
• None is clearly superior to the others.
47
• Assay method should have the ability to measure:
1. Plasma concentrations BELOW ( 0.05 x10ˉ⁶molar).
2. Plasma concentration ABOVE ( 1 x 𝟏𝟎ˉ⁶ molar).
49. • Leucovorin can
compete with methotrexate for the same
transport processes into the cell.
• This is beneficial when treating
methotrexate overdose toxicity.
49
Leucovorin Rescue
• But at the same time is aconsiderable point as
we should start leucvorin rescue after 24 hrs of
methotrexate infusion.
50. Leucovorin Regimen
• The usual regimen of rescue therapy
– leucovorin of ≈ 10 mg/m². (BSA)
– Administered every 6 hours for 72 hours.
• The usual course of rescue therapy is:
– From 12 – 72 hrs.
– Or until the plasma concentration of
methotrexate falls Bellow the critical value
of (0.1x 10⁻⁶ molar).
50
• In some protocols, concentrations of .05x10ˉ⁶ are
considered the value indicating the rescue is complete.
53. 53
Leucovorin Regimen
Clinical Situation Laboratory Finding Leucovorin Dosage
and Duration
Normal
Methotrexate
Elimination
Serum Methotrexate level
approximately
10 x10ˉ⁶molar at 24 hours
after administration
1x10⁻⁶ molar at 48 hours,
and 0.1x10ˉ⁶ at 72 hours
10 mg PO, IM or IV q 6
hours for 60 hours
(10 doses starting at
24 hours after start of
Methotrexate infusion)
54. 54
Leucovorin Regimen
If the methotrexate
concentration falls below
0.1x10ˉ⁶ molar before the
completion of the 72-hour
rescue period.
If the methotrexate
concentrations are still
greater than 0.1x10ˉ⁶
molar at 72-hr but less
than 1x10ˉ⁶ molar at 48
hour.
The rescue can be
discontinued.
The rescue is continued
At dose of 10 mg/m² every
6 hours until the MTX
concentration falls below
0.1x10ˉ⁶ molar.
55. • Methotrexate concentrations in
excess of 1x10ˉ⁶molar at
48 hours are associated with an
increased incidence of methotrexate
toxicity.
• even in face of leucovorin rescue doses
of 10 mgm².
55
56. • When methotrexate concentration
exceeds 1x10ˉ⁶ molar at 48 hours,
increasing the leucovorin rescue
dose to
50 – 100 mgm² or more
reduces methotrexate toxicity.
56
This increased dose enables leucovorin factor to:
• compete successfully with methotrexate for
intracellular transport .
• and to thereby rescue host tissues.
58. Methotrexate plasma leval.
Leucovorin dose regimen.
At 24 hr At 48 hr At 72 hr
10x10ˉ⁶
molar.
1x10ˉ⁶
molar.
0.1x10ˉ⁶
molar.
Normal leucovorin regimen of
10 mgm² q6hr
( till 72 hr).
10x10ˉ⁶
molar.
1x10ˉ⁶
molar.
More than
0.1x10ˉ⁶
molar.
Continue with 10 mgm2 q6hr
(Till methotrexate plasma level reach
0.1x10ˉ⁶ molar).
More than
10x10ˉ⁶
molar.
More than
1x10ˉ⁶
molar.
increasing the leucovorin rescue dose
50 – 100 mgm² or more
(Toxic case). 58
59. Conclusion
• What is MTX ?
• If you have a patient need to
be treated with methotrexate how
can u calculate his LD?
59
• Not all the cases which are treated with
methotrexate need leucovorin rescue,
which cases need the rescue? and why?