How Elahere works and who it is for
Elahere (mirvetuximab soravtansine-gynx) is a folate receptor alpha (FRα) directed antibody and microtubule inhibitor conjugate: an antibody that targets FRα on the cancer cell surface, carrying a microtubule inhibitor payload. Section 1 of the FDA label states the indication: Elahere is for the treatment of adult patients with FRα positive, platinum-resistant epithelial ovarian, fallopian tube, or primary peritoneal cancer, who have received one to three prior systemic treatment regimens. Patient selection comes before any dose: section 2.1 requires selecting patients based on the presence of FRα tumor expression using an FDA-approved test. The full generic name matters for pharmacy safety: the product is mirvetuximab soravtansine-gynx, and the dosing weight for this drug is unusual, which is exactly why this calculator exists. Elahere is one of the few oncology products whose label dose is calculated on adjusted ideal body weight, using a formula printed in the label itself, rather than on actual body weight or body surface area.
The labelled dose and the AIBW formula (label section 2.2)
Section 2.2 gives the recommended dosage: 6 mg/kg adjusted ideal body weight (AIBW), administered once every 3 weeks (a 21-day cycle) as an intravenous infusion, until disease progression or unacceptable toxicity. The label explains the reason in one sentence: dosing based on AIBW reduces exposure variability for patients who are either underweight or overweight. The total dose is calculated from each patient's AIBW using the formula printed in the label:
AIBW = IBW + 0.4 x (actual weight in kg minus IBW), where female IBW in kg = 0.9 x height in cm minus 92.
Two details of that formula are easy to get wrong, and both are handled exactly as the label prints them. First, the IBW equation is the label's own height equation for women, 0.9 times height in centimetres minus 92. It is not the Devine equation and it has no separate male version, because Elahere is indicated in a female patient population. Second, the 0.4 factor applies in both directions: a patient heavier than her IBW is dosed on her IBW plus 40% of the excess, and a patient lighter than her IBW is dosed on her IBW minus 40% of the deficit, so the AIBW always sits between actual weight and IBW, pulled 60% of the way toward the IBW. This calculator implements the formula in that order and shows the IBW and AIBW it used, so the arithmetic can be checked against the label at the bedside or in the pharmacy.
Worked example: the label's own numbers
The worked example used in the Elahere product information is a female patient 165 cm tall weighing 80 kg. First the IBW: 0.9 x 165 = 148.5, minus 92, gives an IBW of 56.5 kg. Then the AIBW: 56.5 + 0.4 x (80 minus 56.5) = 56.5 + 9.4 = 65.9 kg. At the starting dose of 6 mg/kg, the dose is 6 x 65.9 = 395.4 mg. Elahere solution is 5 mg/mL (100 mg in 20 mL), so the volume to withdraw is 395.4 divided by 5 = 79.08 mL, which needs 4 of the 100 mg vials, with 4.6 mg left over and discarded because each vial is single-dose. The same patient at the two reduction levels and several other heights and weights:
| Height | Actual weight | IBW | AIBW | Dose at 6 mg/kg | Volume at 5 mg/mL | 100 mg vials |
|---|---|---|---|---|---|---|
| 160 cm | 47 kg | 52.0 kg | 50.0 kg | 300.0 mg | 60.00 mL | 3 |
| 165 cm | 50 kg | 56.5 kg | 53.9 kg | 323.4 mg | 64.68 mL | 4 |
| 165 cm | 80 kg | 56.5 kg | 65.9 kg | 395.4 mg | 79.08 mL | 4 |
| 170 cm | 70 kg | 61.0 kg | 64.6 kg | 387.6 mg | 77.52 mL | 4 |
Compare the first and third rows to see what AIBW does. The 80 kg patient is not dosed on 80 kg, which would be 480 mg; she is dosed on 65.9 kg, 395.4 mg. The 47 kg patient is not dosed on 47 kg either; her AIBW of 50.0 kg is above her actual weight because her IBW of 52 kg is above it, and the formula pulls her dose up toward the ideal weight rather than down with the scale. That is the exposure-variability smoothing the label describes.
Dose reductions and modifications for adverse reactions (label section 2.4)
Table 2 of the label is the dose reduction schedule. The first dose reduction is 5 mg/kg AIBW once every 3 weeks and the second dose reduction is 4 mg/kg AIBW once every 3 weeks. There is no third step: Elahere is permanently discontinued in patients who cannot tolerate 4 mg/kg AIBW. The calculator's dose level selector covers all three levels using the same AIBW, so a reduced dose can be calculated for the same patient without redoing the weight arithmetic. Table 3 then maps each adverse reaction to an action:
| Adverse reaction | Severity | Label action |
|---|---|---|
| Keratitis / keratopathy | Nonconfluent superficial keratitis | Monitor |
| Keratitis / keratopathy | Confluent superficial keratitis, a corneal epithelial defect, or 3-line or more loss in best corrected visual acuity | Withhold until improved or resolved, then maintain at the same dose level or consider dose reduction |
| Keratitis / keratopathy | Corneal ulcer or stromal opacity, or best corrected distance visual acuity 20/200 or worse | Withhold until improved or resolved, then reduce by one dose level |
| Keratitis / keratopathy | Corneal perforation | Permanently discontinue |
| Uveitis | Grade 1 (rare cell in anterior chamber) | Monitor |
| Uveitis | Grade 2 (1 to 2+ cell or flare in anterior chamber) | Withhold until Grade 1 or less, then maintain dose at the same dose level |
| Uveitis | Grade 3 (3+ cell or flare in anterior chamber) | Withhold until Grade 1 or less, then reduce dose by one dose level |
| Uveitis | Grade 4 (hypopyon) | Permanently discontinue |
| Pneumonitis | Grade 1 | Monitor |
| Pneumonitis | Grade 2 | Withhold until Grade 1 or less, then maintain at the same dose level or consider dose reduction |
| Pneumonitis | Grade 3 or 4 | Permanently discontinue |
| Peripheral neuropathy | Grade 2 | Withhold until Grade 1 or less, then reduce by one dose level |
| Peripheral neuropathy | Grade 3 or 4 | Permanently discontinue |
| Infusion-related reactions / hypersensitivity | Grade 1 | Maintain infusion rate |
| Infusion-related reactions / hypersensitivity | Grade 2 | Interrupt the infusion and give supportive treatment; after recovery, resume at 50% of the previous rate and increase as appropriate; give additional premedication for future cycles |
| Infusion-related reactions / hypersensitivity | Grade 3 or 4 | Immediately stop the infusion, give supportive treatment, and permanently discontinue |
| Hematological | Grade 3 or 4 | Withhold until Grade 1 or less, then resume at one lower dose level |
| Other adverse reactions | Grade 3 | Withhold until Grade 1 or less, then resume at one lower dose level |
| Other adverse reactions | Grade 4 | Permanently discontinue |
Grades are NCI CTCAE version 5.0 unless otherwise specified. These decisions are clinical decisions made with the patient and the eye examination in front of the prescriber; the table is reproduced here in full so that the dose level chosen in the calculator matches the action the label prescribes.
Premedication and required eye care (label section 2.3)
Table 1 of the label sets the premedication given before each infusion to reduce infusion-related reactions, nausea and vomiting: a corticosteroid (intravenous dexamethasone 10 mg), an antihistamine (oral or intravenous diphenhydramine 25 mg to 50 mg) and an antipyretic (oral or intravenous acetaminophen 325 mg to 650 mg), all at least 30 minutes prior to the infusion, plus an antiemetic (a 5-HT3 serotonin receptor antagonist or appropriate alternatives) before each dose and thereafter as needed. Additional premedication, including corticosteroids the day prior, is considered for patients who experienced infusion-related reactions. The eye programme is part of the dosage section, not an optional extra: an ophthalmic exam including visual acuity and slit lamp exam before initiation, every other cycle for the first 8 cycles, and as clinically indicated; ophthalmic topical steroids, one drop in each eye 6 times daily starting the day prior to each infusion until day 4, then 4 times daily for days 5 to 8 of each cycle, with the initial prescription and renewals made only after a slit lamp examination; and lubricating eye drops at least four times daily and as needed, instilled at least 10 minutes after the steroid drops.
Ocular toxicity, pneumonitis and peripheral neuropathy in the label's numbers (sections 5.1 to 5.3)
Section 5.1 records that ocular adverse reactions occurred in 59% of patients with ovarian cancer treated with Elahere; 11% experienced Grade 3 ocular reactions and two patients (0.3%) had Grade 4 events (keratopathy and cataract). The most common ocular reactions were blurred vision (48%), keratopathy (36%), dry eye (27%), cataract (16%), photophobia (14%) and eye pain (10%). The median time to onset of the first ocular reaction was 5.1 weeks; of patients with ocular events, 53% had complete resolution and 38% had partial improvement at last follow up, and ocular reactions led to permanent discontinuation in 1%. Patients are advised to avoid contact lenses during treatment unless directed by a healthcare provider, and to be referred promptly to an eye care professional for new or worsening symptoms. Section 5.2 covers pneumonitis: severe, life-threatening or fatal interstitial lung disease including pneumonitis can occur; pneumonitis occurred in 10% of patients, including 1% Grade 3 and one Grade 4 event, and led to permanent discontinuation in 3%. Section 5.3 covers peripheral neuropathy, which occurred in 36% of patients across trials, with 3% Grade 3; the median onset was 5.9 weeks. Section 5.4 adds the embryo-fetal toxicity warning: Elahere can cause fetal harm, and patients are advised of the risk and to use effective contraception.
Preparation and administration in full (label section 2.5)
Elahere is a hazardous drug and the label's preparation steps are specific. Calculate the dose in milligrams from the patient's AIBW, the total volume in millilitres of solution required, and the number of vials needed; more than one vial will be needed for a full dose. The vials are 100 mg in 20 mL (5 mg/mL), single-dose, with no preservatives; discard any unused drug remaining in the vial. Warm the vials to room temperature, swirl gently and inspect, and do not shake. The dose must be diluted in 5% Dextrose Injection, USP to a final concentration of 1 mg/mL to 2 mg/mL; for the 395.4 mg worked example that means a final diluted volume between 197.7 mL and 395.4 mL, which is why the calculator shows that range. Elahere is incompatible with 0.9% Sodium Chloride Injection and must not be mixed with any other drugs or intravenous fluids. If not used immediately, the diluted solution may be stored at ambient temperature (18 to 25 degrees C) for no more than 8 hours including infusion time, or refrigerated at 2 to 8 degrees C for no more than 24 hours, then administered within 8 hours after reaching room temperature; do not freeze it. Administration is intravenous infusion only, through a 0.2 or 0.22 micron polyethersulfone (PES) in-line filter, with no substitute membrane materials. The initial dose starts at 1 mg/min; if well tolerated after 30 minutes the rate can increase to 3 mg/min, and after a further 30 minutes to 5 mg/min. Later infusions start at the maximally tolerated rate, up to a maximum of 5 mg/min. After the infusion, flush the line with 5% Dextrose Injection, USP, and no other fluid, to deliver the full dose.
Sources
ELAHERE (mirvetuximab soravtansine-gynx) injection, for intravenous use. FDA-approved prescribing information, ImmunoGen, Inc. (DailyMed, setid 00c424b5-6ccd-48ab-9e88-1986451120e2): boxed warning on ocular toxicity; section 1 indications (FRα positive, platinum-resistant epithelial ovarian, fallopian tube or primary peritoneal cancer after one to three prior systemic regimens); section 2.1 patient selection by FRα tumor expression using an FDA-approved test; section 2.2 recommended dosage (6 mg/kg AIBW once every 3 weeks until disease progression or unacceptable toxicity; AIBW = IBW + 0.4 x (actual weight minus IBW); female IBW = 0.9 x height in cm minus 92); section 2.3 premedication Table 1 (dexamethasone 10 mg intravenous, diphenhydramine 25 to 50 mg, acetaminophen 325 to 650 mg, at least 30 minutes prior; antiemetic before each dose) and the ophthalmic exam, topical steroid and lubricating drop schedule; section 2.4 Table 2 dose reductions (first 5 mg/kg AIBW, second 4 mg/kg AIBW, permanently discontinue if 4 mg/kg cannot be tolerated) and Table 3 dosage modifications for adverse reactions; section 2.5 preparation and administration (100 mg/20 mL, 5 mg/mL single-dose vials; dilute in 5% Dextrose Injection, USP to 1 to 2 mg/mL; incompatible with 0.9% sodium chloride; 0.2 or 0.22 micron PES in-line filter; infusion rate 1 mg/min increasing to 3 and then 5 mg/min; flush with 5% dextrose); section 3 dosage forms; sections 5.1 to 5.4 on ocular disorders, pneumonitis, peripheral neuropathy and embryo-fetal toxicity.