CD20×CD3 Bispecific T-Cell Engager Pregnancy: May cause fetal harm based on mechanism of action. There are no available data in pregnant women; no animal reproductive or developmental toxicity studies have been conducted. Mosunetuzumab causes T-cell activation and cytokine release, and immune activation may compromise pregnancy maintenance; based on CD20 expression on B-cells it can cause B-cell lymphocytopenia in infants exposed in utero. Human IgG crosses the placenta. Advise women of the potential risk to the fetus and to use effective contraception.

Mosunetuzumab

Brand names: Lunsumio

Mosunetuzumab is a bispecific monoclonal antibody used to treat relapsed or refractory follicular lymphoma after prior lines of therapy.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Step-up dosing in 21-day cycles: Cycle 1 — 1 mg on Day 1, 2 mg on Day 8, 60 mg on Day 15; Cycle 2 — 60 mg on Day 1; Cycles 3 and onwards — 30 mg on Day 1

Route: Intravenous infusion only, administered through a dedicated infusion line. Do not use an in-line filter; drip chamber filters can be used. Administer to well-hydrated patients.

Frequency: 21-day treatment cycles. Cycle 1 doses on Days 1, 8 and 15; thereafter Day 1 of each cycle.

Infusion rate: administer Cycle 1 doses over a minimum of 4 hours; from Cycle 2 administer over 2 hours if infusions from Cycle 1 were well tolerated. Duration: administer for 8 cycles unless unacceptable toxicity or disease progression; patients achieving a complete response need no further treatment beyond 8 cycles; patients with a partial response or stable disease after 8 cycles should receive an additional 9 cycles (17 cycles total). Premedication (§2.3): in Cycles 1 and 2 all patients receive a corticosteroid (dexamethasone 20 mg IV preferred, or methylprednisolone 80 mg IV) completed at least 1 hour before infusion, an antihistamine (diphenhydramine hydrochloride 50 mg to 100 mg or equivalent oral/IV antihistamine) and an antipyretic (oral acetaminophen/paracetamol 500 mg to 1,000 mg) at least 30 minutes before infusion; from Cycle 3 the same premedication is given to patients who experienced any grade CRS with the previous dose. Product-confusion warning: LUNSUMIO and LUNSUMIO VELO have different dosage and administration instructions — check the product label and do not substitute one for the other; this record reflects LUNSUMIO for intravenous infusion only. A detailed dose-delay restart table (§2.2, Table 2) governs re-initiation after missed doses and is not reproduced here. Should only be administered by a qualified healthcare professional with medical support to manage severe reactions such as CRS and neurologic toxicity including ICANS. Paediatric use: the US label states safety and efficacy have not been established in paediatric patients — no paediatric dose is given; verify any paediatric use against a children's formulary and specialist protocol. No UK SPC (eMC) record was present in the source bundle — this dose is from US prescribing information and must be verified against UK labelling.

Dose auto-extracted from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

None (US label §4 states: None)

Side effects

Cytokine release syndrome (CRS)
Fatigue
Rash and pruritus
Headache and pyrexia
Musculoskeletal pain, cough and peripheral neuropathy
Grade 3-4 laboratory abnormalities: decreased lymphocyte count, decreased phosphate, increased glucose, decreased neutrophil count, increased uric acid, decreased haemoglobin, decreased platelets

Interactions

CYP450 substrates: mosunetuzumab causes release of cytokines that may suppress CYP450 enzyme activity, resulting in increased exposure of CYP450 substrates. Increased exposure is more likely after the first dose on Cycle 1 Day 1 and up to 14 days after the 60 mg dose on Cycle 2 Day 1, and during and after CRS. Monitor for toxicity or concentrations of CYP450 substrate drugs where minimal concentration changes may lead to serious adverse reactions; consult the concomitant drug's prescribing information for dosage modification.

Clinical monograph

How it works

It is a CD20 x CD3 bispecific antibody that simultaneously engages CD20 on B lymphoma cells and CD3 on T cells, redirecting T-cell-mediated killing of the malignant B cells.

Prescribing in practice

Cytokine release syndrome is an important risk, so step-up dosing is used at initiation and patients must be monitored and managed according to the SPC.
Serious infections and neurological adverse effects can occur and require prompt recognition.
It should be administered in a setting equipped to manage cytokine release syndrome by an experienced specialist team.

Monitoring

Monitor closely for cytokine release syndrome, neurological symptoms, infection and tumour lysis, particularly during early cycles.

Counselling the patient

Higher first doses are given gradually to lower the risk of a reaction.
Report fever, confusion, breathlessness or feeling very unwell straight away.

Evidence & guidelines

Activity in relapsed or refractory follicular lymphoma is supported by its pivotal phase 1/2 trial and reflected in MHRA approval.

Reference: CELESTIMO trial (Budde et al. Lancet 2022); NICE TA900; MHRA SPC Lunsumio; GO29781 trial (Budde et al. JCO 2022); Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways