Aromatase (IC50 = 11.5 nM)

In a dose- and time-dependent manner, letrozole (0.1–100 nM; 24-96 hours) strongly suppresses the development of MCF-7 epithelial breast cancer cells [2]. The stimulating impact of testosterone on MCF-7 cell growth is considerably inhibited by letrozole (10 nM) [2]. In MCF-7 cells, letrozole (10 nM; 24-48 hours) reduces the amounts of released metalloproteinases (MMP-2 and MMP-9) [2].

Rats treated with letrozole (3–300 μg/kg; once daily oral gavage for six weeks) show anti-tumor effects[3].

---

In vivo, in ACTH-treated rats, CGS 20267 does not affect plasma levels of corticosterone or aldosterone at a dose of 4 mg/kg p.o. (1000 times higher than the ED50 for aromatase inhibition in vivo). In adult female rats, a 14-day treatment with 1 mg/kg p.o. daily, completely interrupts ovarian cyclicity and suppresses uterine weight to that seen 14 days after ovariectomy. In adult female rats bearing estrogen-dependent DMBA-induced mammary tumors, 0.1 mg/kg p.o. given daily for 42 days caused almost complete regression of tumors present at the start of treatment. Thus compared to each other, CGS 16949A and CGS 20267 are both highly potent in inhibiting estrogen biosynthesis in vitro and in vivo. The striking difference between them is that unlike CGS 16949A, CGS 20267 does not affect adrenal steroidogenesis in vitro or in vivo, at concentrations and doses several orders of magnitude higher than those required to inhibit estrogen biosynthesis[1].

CGS 20267 is a new non-steroidal compound which potently inhibits aromatase in vitro (IC50 of 11.5 nM) and in vivo (ED50 of 1-3 micrograms/kg p.o.), CGS 20267 maximally inhibits estradiol production in vitro in LH-stimulated hamster ovarian tissue at 0.1 microM with an IC50 of 0.02 microM and does not significantly affect progesterone production up to 350 microM. In ACTH-stimulated rat adrenal tissue in vitro, aldosterone production was inhibited with an IC50 of 210 microM (10,000 times higher than the IC50 for estradiol production); no significant effect on corticosterone production was seen at 350 microM[1].

Cell Viability Assay[2]
Cell Types: MCF-7 Cell
Tested Concentrations: 0.1, 1, 10, 100 nM
Incubation Duration: 24, 48, 96 hrs (hours)
Experimental Results: Inhibition of cell growth in a dose- and time-dependent manner.

Animal/Disease Models: Adult female rats bearing mammary tumors[3]
Doses: 3, 10, 30, 100, 300 μg/kg
Route of Administration: po (oral gavage) one time/day for 6 weeks
Experimental Results: Induced complete regression of mammary tumors, with an ED50 of 10-30 μg/kg/day.

Absorption, Distribution and Excretion
Letrozole is 99.9% orally bioavailable. A 2.5mg oral dose reaches a Cmax of 104nmol/L with a Tmax of 8.10h, and an AUC of 7387nmol\*h/L.
Letrozole is 90% eliminated in the urine. 75% of the dose is recovered as a glucuronide metabolite, 9% is in the form of the ketone and carbinol metabolites, and 6% is recovered in urine as unchanged letrozole.
The volume of distribution of letrozole is 1.87L/kg.
The average clearance after a single dose of letrozole was 1.52L/h and at steady state was 1.20L/h.
Letrozole is rapidly and completely absorbed from the GI tract following oral administration. Steady-state plasma concentrations of the drug are reached in 2-6 weeks in patients receiving letrozole 2.5 mg daily. Letrozole exhibits slightly nonlinear pharmacokinetics with repeated administration of 2.5 mg daily, with steady-state plasma concentrations 1.5-2 times higher than predicted based on plasma concentrations measured after a single dose. However, continuous accumulation of letrozole does not occur, and steady-state concentrations are maintained over extended periods of daily drug administration. Food does not affect the oral absorption of the drug.
Letrozole has a large volume of distribution of approximately 1.9 L/kg. Letrozole is weakly bound to plasma proteins.
Following oral administration of radiolabeled letrozole, 90% of the administered dose was excreted in the urine. Of the radiolabeled drug recovered in urine, at least 75% was the glucuronide of the carbinol metabolite, about 9% consisted of 2 unidentified metabolites, and 6% was unchanged drug.
It is not known whether letrozole is distributed into human breast milk.
For more Absorption, Distribution and Excretion (Complete) data for LETROZOLE (6 total), please visit the HSDB record page.

---

Metabolism / Metabolites
Letrozole is metabolized by CYP2A6 to a ketone analog metabolite, which is further metabolized by CYP3A4 and CYP2A6 to 4,4'-(hydroxymethylene)dibenzonitrile. 4,4'-(hydroxymethylene)dibenzonitrile is glucuronidated by UGT2B7.
The primary elimination pathway of letrozole consists of slow metabolism in the liver to a pharmacologically inactive carbinol metabolite (4,4'-methanol-bisbenzonitrile) followed by renal excretion of the glucuronide conjugate of this metabolite. Formation of the carbinol metabolite is mediated by cytochrome P-450 (CYP) isoenzymes 3A4 and 2A6, and formation of the ketone analog of the carbinol metabolite is mediated by isoenzyme 2A6.
Primarily hepatic via CYP3A4 and CYP2A6. Letrozole inhibits the aromatase enzyme by competitively binding to the heme of the cytochrome P450 subunit of the enzyme, resulting in a reduction of estrogen biosynthesis in all tissues. It is metabolized slowly to an inactive metabolite whose glucuronide conjugate is excreted renally, representing the major clearance pathway.
Half Life: 2 days

---

Biological Half-Life
The terminal elimination half life of letrozole is approximately 42h in healthy volunteers, but longer in breast cancer patients.
Letrozole has a terminal elimination half-life of about 2 days.

Toxicity Summary
Letrozole is a nonsteroidal competitive inhibitor of the aromatase enzyme system; it inhibits the conversion of androgens to estrogens. In adult nontumor- and tumorbearing female animals, letrozole is as effective as ovariectomy in reducing uterine weight, elevating serum Leuteinizing hormone (LH), and causing the regression of estrogen-dependent tumors. In contrast to ovariectomy, treatment with letrozole does not lead to an increase in serum (folicile stimulating hormone (FSH). Letrozole selectively inhibits gonadal steroidogenesis but has no significant effect on adrenal mineralocorticoid or glucocorticoid synthesis. Organic nitriles decompose into cyanide ions both in vivo and in vitro. Consequently the primary mechanism of toxicity for organic nitriles is their production of toxic cyanide ions or hydrogen cyanide. Cyanide is an inhibitor of cytochrome c oxidase in the fourth complex of the electron transport chain (found in the membrane of the mitochondria of eukaryotic cells). It complexes with the ferric iron atom in this enzyme. The binding of cyanide to this cytochrome prevents transport of electrons from cytochrome c oxidase to oxygen. As a result, the electron transport chain is disrupted and the cell can no longer aerobically produce ATP for energy. Tissues that mainly depend on aerobic respiration, such as the central nervous system and the heart, are particularly affected. Cyanide is also known produce some of its toxic effects by binding to catalase, glutathione peroxidase, methemoglobin, hydroxocobalamin, phosphatase, tyrosinase, ascorbic acid oxidase, xanthine oxidase, succinic dehydrogenase, and Cu/Zn superoxide dismutase. Cyanide binds to the ferric ion of methemoglobin to form inactive cyanmethemoglobin. (L97)

---

Hepatotoxicity
Serum enzymes are reported to be elevated in up to 1% of women treated with letrozole, but these elevations are usually mild, asymptomatic and self-limited, rarely requiring dose modification. There have been few published instances of clinically apparent liver injury associated with long term letrozole therapy. More frequent have been reports of cholestatic and hepatocellular liver injury associated with anastrozole and exemestane, typically arising after 1 to 4 months of therapy and presenting with jaundice. While cases have been severe, recovery is usually prompt once the agent is stopped. There have been no cases of severe jaundice, acute liver failure, chronic hepatitis or vanishing bile duct syndrome attributed to letrozole use. Unlike tamoxifen, letrozole has not been associated with development of fatty liver disease, steatohepatitis or cirrhosis.
Likelihood score: D (possible rare cause of clinically apparent liver injury).

---

Effects During Pregnancy and Lactation
◉ Summary of Use during Lactation
No information is available on the use of letrozole during breastfeeding. The manufacturer recommends that breastfeeding be discontinued during letrozole therapy and for 3 weeks after the last dose.
◉ Effects in Breastfed Infants
Relevant published information was not found as of the revision date.
◉ Effects on Lactation and Breastmilk
Relevant published information was not found as of the revision date.

---

Protein Binding
Letrozole is 60% bound to proteins. 55% is bound to albumin.

---

Interactions
Because metabolism of letrozole is mediated by cytochrome P-450 (CYP) isoenzymes 3A4 and 2A6, agents that induce or inhibit these isoenzymes may alter the metabolism of the drug. Cimetidine, which inhibits hepatic microsomal enzymes, did not alter the pharmacokinetics of letrozole. Results of an in vitro study did not show inhibition of letrozole metabolism by diazepam.
Because estrogens may diminish the pharmacologic action of aromatase inhibitors, such as letrozole, these agents should not be used concomitantly.
Concomitant use of tamoxifen 20 mg daily and letrozole 2.5 mg daily reduced letrozole plasma concentrations by an average of 38%. In a separate study, no effect of letrozole on the pharmacokinetics of tamoxifen, its principal active metabolite, N-desmethyltamoxifen, or 4-hydroxytamoxifen was observed. Analysis of blood samples from both of these studies demonstrates similar degrees of estrogen suppression for letrozole alone and in combination with tamoxifen. ... The concomitant use of letrozole and tamoxifen is not recommended.
Twelve of 17 patients completed the core period of the trial in which 2.5 mg/day letrozole was administered alone for 6 weeks and in combination with 20 mg/day tamoxifen for the subsequent 6 weeks. Patients responding to treatment continued on the combination until progression of disease or any other reason for discontinuation. ... Marked suppression of estradiol, estrone, and estrone sulfate occurred with letrozole treatment, and this was not significantly affected by the addition of tamoxifen. However, plasma levels of letrozole were reduced by a mean 37.6% during combination therapy (P<0.0001), and this reduction persisted after 4-8 months of combination therapy. Letrozole is the first drug to be described in which this pharmacokinetic interaction occurs with tamoxifen. The mechanism is likely to be a consequence of an induction of letrozole-metabolizing enzymes by tamoxifen but was not further addressed in this study. It is possible that the antitumor efficacy of letrozole may be affected. Thus, sequential therapy may be preferable with these two drugs.

[1]. Highly selective inhibition of estrogen biosynthesis by CGS 20267, a new non-steroidal aromatase inhibitor. J Steroid Biochem Mol Biol. 1990 Dec 20;37(6):1021-7.

[2]. Letrozole as a potent inhibitor of cell proliferation and expression of metalloproteinases (MMP-2 and MMP-9) by human epithelial breast cancer cells. Int J Cancer. 2003 Mar 20;104(2):155-60.

[3]. Anti-tumor and endocrine effects of non-steroidal aromatase inhibitors on estrogen-dependent rat mammary tumors. J Steroid Biochem Mol Biol. 1993 Mar;44(4-6):633-6.

Therapeutic Uses
Antineoplastic
Letrozole is indicated for first-line treatment of postmenopausal women with hormone receptor positive or hormone receptor unknown locally advanced or metastatic breast cancer. Letrozole is also indicated for treatment of advanced breast cancer in postmenopausal women with disease progression following antiestrogen therapy. /Included in US product label/

---

Drug Warnings
A 37-year-old premenopausal woman with relapsed breast cancer (BC) in the right supraclavicular nodes, after failed treatment with the combination luteinizing hormone releasing hormone-a (LHRHa; triptorelin) plus tamoxifen, was started on triptorelin 3.75 mg every 28 days plus letrozole 2.5 mg daily. Approximately 6 months after starting this therapy, she complained of a daily scalp hair loss while combing and progressively developed a diffuse non-scarring alopecia on her crown. There were no signs of virilization ... She was not taking any other drug. Hematological parameters were normal. Blood examination ruled out pituitary or thyroid problems. There were no other possible causes that could induce alopecia, such as lupus erythematosus, HIV infection, secondary syphilis, or deficiencies of protein, iron, biotin or zinc.
In patients receiving letrozole as first-line therapy, bone pain, back pain, and limb pain occurred in 22, 18, and 10% of patients, respectively. In patients receiving letrozole as second-line therapy, adverse musculoskeletal effects (including musculoskeletal pain, skeletal pain, back pain, arm pain, and leg pain) were reported in 21% and fracture was reported in less than 5% of patients. Arthralgia was reported in 16% of patients receiving letrozole as first-line therapy and in 8% of patients receiving the drug as second-line therapy. Hypercalcemia occurred in less than 5% of patients receiving letrozole as second-line therapy.
Adverse musculoskeletal effects have been reported in patients receiving letrozole as adjuvant therapy for early-stage breast cancer in clinical trials. In a double-blind, randomized trial in postmenopausal women with hormone receptor-positive breast cancer who had received approximately 5 years of tamoxifen adjuvant therapy following primary treatment for early breast cancer, extended adjuvant therapy with letrozole was associated with an increased incidence of arthritis, arthralgia, and myalgia, and a trend toward higher rates of newly diagnosed osteoporosis and bone fracture compared with placebo therapy.
All women receiving adjuvant therapy with letrozole should be advised to adopt lifestyle changes (eg, weight-bearing exercise, abstinence from smoking, moderation in alcohol consumption) and dietary supplementation with calcium and vitamin D to reduce the risk of osteoporosis.
For more Drug Warnings (Complete) data for LETROZOLE (26 total), please visit the HSDB record page.

---

Pharmacodynamics
Letrozole is an aromatase inhibitor used in the treatment of breast cancer. Aromatase inhibitors work by inhibiting the action of the enzyme aromatase, which converts androgens into estrogens by a process called aromatization. As breast tissue is stimulated by estrogens, decreasing their production is a way of suppressing recurrence of the breast tumor tissue. Letrozole is a third generation type II aromatase inhibitor used to treat estrogen dependant breast cancers. It has a long duration of action as it has a half life of over 42 hours in breast cancer patients. Patients should be counselled regarding the risk of interstitial lung disease, pneumonitis, QT prolongation, elevated transaminase levels, neutropenia, and embryo-fetal toxicity.

C17H11N5

285.3

263.142

C, 71.57; H, 3.89; N, 24.55

112809-51-5

Letrozole-d4;1133712-96-5

3902

White to yellowish crystalline powder

1.1±0.1 g/cm3

472.0±55.0 °C at 760 mmHg

181-183ºC

214.2±24.5 °C

0.0±1.2 mmHg at 25°C

1.615

3.7

0

4

3

22

420

0

N1(C([H])=NC([H])=N1)C([H])(C1C([H])=C([H])C(C#N)=C([H])C=1[H])C1C([H])=C([H])C(C#N)=C([H])C=1[H]

HPJKCIUCZWXJDR-UHFFFAOYSA-N

InChI=1S/C17H11N5/c18-9-13-1-5-15(6-2-13)17(22-12-20-11-21-22)16-7-3-14(10-19)4-8-16/h1-8,11-12,17H

4-[(4-cyanophenyl)-(1,2,4-triazol-1-yl)methyl]benzonitrile

Abbreviation; CGS 20267; CGS20267; CGS-20267; LTZ; Trade name: Femara; Letoval; Femara; 4,4'-((1h-1,2,4-triazol-1-yl)methylene)dibenzonitrile; Letrozol;

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

Solubility in Formulation 1: ≥ 2.08 mg/mL (7.29 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 20.8 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

---

Solubility in Formulation 2: ≥ 2.08 mg/mL (7.29 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 20.8 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

---

View More

Solubility in Formulation 3: ≥ 2.08 mg/mL (7.29 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 20.8 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

---

Solubility in Formulation 4: 0.5% CMC: 10 mg/mL

---

 (Please use freshly prepared in vivo formulations for optimal results.)