Methotrexate is a folic acid anti-metabolite that acts by inhibiting dihydrofolate reductase. It is used to treat cancers and autoimmune diseases at both low and high doses. Monitoring methotrexate levels is important to guide leucovorin rescue dosing and prevent toxicity. Methotrexate has a short initial half-life of 3 hours and longer terminal half-life of 10 hours. It is primarily cleared renally, though clearance can be reduced by various drug interactions. Therapeutic methotrexate levels are above 10-7 molar for less than 48 hours, while toxic levels exceed 10-7 molar for over 48 hours or are above 10-6 molar

1. CPK of
Methotrexate

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

5. 5
Mode of Action

6. Uses & doses
• MTX's doses are use-dependent.
• The primary setting for pharmacokinetic
monitoring of MTX is its use in
high doses.
6

7. 7

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

12. 12
Side effects

13. 13
Side effects

14. 14
Side effects

15. 15
Side effects

16. 16
Side effects

17. Drug Interaction
17

18. Drug Interaction Mechanism Management
Aminoglycosides
(Oral)
Decreased
methotrexate levels
Decreased GI
absorption of
methotrexate
Parenteral use
Corticosteroids Decrease
methotrexate levels
Decrease the uptake
of methotrexate into
leukemia cells
Separate 12 hours;
dexamethasone affect
methotrexate uptake
into cells
NSAIDs Increased
methotrexate levels
Reduced renal
clearance of
methotrexate
Monitor methotrexate
levels, risk may be
lower with selective
COX-2 inhibitors
Omeprazole Rapid, major, possible;
increased
methotrexate levels
Reduced renal
clearance of
methotrexate
Use H2 antagonist
(e.g., ranitidine)
18

19. 19
Drug Interaction Mechanism Management
Probenecid Rapid, major, probable;
increased methotrexate
levels
Decreased renal
excretion of MTX
Primarily a concern with
HDMTX; monitor MTX
levels i.e., longer
leucovorin rescue
Salicylates Rapid, major, suspected;
increased methotrexate
levels
Decreased renal
clearance and plasma
protein binding of
methotrexate
Salicylate doses used for
prophylaxis of CV events
are not likely to be a
concern; consider
monitoring MTX levels
Sulfonamides (e.g., co-
trimoxazole,
sulfamethoxazole,
sulfisoxazole)
Delayed, major,
suspected; increased
methotrexate levels
Decreased protein
binding and renal
clearance of MTXfolate
deficiencyacute
megaloblastic anemia
Primarily a concern with
high-dose methotrexate;
monitor methotrexate
levels i.e., longer
leucovorin rescue

20. Units Conversion
• Methotrexate concentration can
be reported in units of :
– MgL
– Molar
– Micro-molar
20

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]

24. Bioavailability
According
to the dose
< 30 mg > 30 mg
24

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 )

32. 0.1
1
10
100
1000
10000
0 12 24 36 48 60 72
MTX
conc.
(µmol/L)
Time 32

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)

40. Clearance (Cl)
ClCr for males (ml/min) =
(140−𝐴𝑔𝑒)(𝑊𝑒𝑖𝑔ℎ𝑡)
72 𝑆𝐶𝑟𝑠𝑠
.. (ml/min)
𝐶𝑙𝑀𝑇𝑋 = (1.6)(𝐶𝑙𝑐𝑟 [𝐿 ℎ𝑟] ) ….. (L/hr)
40

41. Steady State:
Css ave =
(𝑆)(𝐹)(𝐷𝑜𝑠𝑒 𝑚𝑔
𝜏 ℎ𝑟 )
𝐶𝑙
..(mg/L)
41
Loading Dose
LD = Cpss x Vd (0.2-0.5 L/Kg) .. (mg)

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).

48. Leucovorin
48

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.

51. Leucovorin Rescue Schedules
Following Treatment With
HDMTX
51

52. 52
10
0.1
1

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.

57. 1.00E-08
1.00E-07
1.00E-06
1.00E-05
1.00E-04
0 12 24 36 48 60 72 84 96 108
Conc.
(
µmol)
Time (hrs)
MTX Conc (Molar)
24
36
48
60
84
96
60
72
78
57

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?