BITES

Question 1

blincyto in vitro

Answer 1

Dreier et al (2002)
demonstrated high efficacy of blinatumomab in vitro
• Found that majority of T cells could induce half-maximal B-lymphoma cell lysis when treated with only 10-50pg/ml of blinatumomab
• Efficacy was 100,000-fold greater than that of rituximab

Question 2

blinatumomab in vivo (mice)

Answer 2

Dreier et al (2003)
• Treated NOD/SCID mice, inoculated with human PBMC and B cell lymphoma cells, with anti-CD19xCD3 BITE
• Untreated mice inoculated with B lymphoma cells, with or without PBMCs, all developed palpable tumours within 30 days which rapidly increased in size, all mice required euthanisation
• Treatment with BITE (0.5 or 5ug) significantly reduced tumour growth and improved tumour-free survival = less effective when treatment was delayed
• BITE also effective against established, disseminated blood-borne tumours = 50% of animals treated with 3 doses of 5ug survived beyond 80-day observation period without symptoms, all control animals died within 45 days
• Limitations of study
- Had to infuse human T cells  limited in number, not pre-stimulated so adhesion and extravasation to distant tumour sites may be limited, short half-life which may explain reduced efficacy when treatment was delayed  results may underestimate potential of BITE in humans

Question 3

blinatumomab in humans

Answer 3

Kantarijan et al (2017)
• Open-label, phase 2 clinical trial
• 376 patients with heavily pretreated B cell ALL randomised to blinatumomab or standard chemotherapy
• Blinatumomab was associated with
1. significantly prolonged overall survival (7.7 months vs 4.0 months)
2. significantly higher complete remission rates (34% vs 16%)
3. longer event-free survival (7.3 months vs 4.6 months)
4. similar incidence of adverse effects as chemotherapy arm

Question 4

pros and cons

Answer 4

Advantages
Off-the-shelf therapy = doesn’t need to be personalised for each individual patient  cheaper, easier, more rapidly available

Bypass MHC I  T cells can still mount a sufficient response against cancer cells that have downregulated MHC expression

Disadvantages
Only target cell surface antigens (10% of expressed genome)
Requires engagement of 1000s of molecules by antigen-recognition domain for robust response

Off-target effects if targeted antigen is expressed on healthy cells = see Blincyto

T cell activation elicits potent pro-inflammatory cytokine release = side effects

Short half-life = must be continuously infused

Question 5

what are ImmTacs

Answer 5

Immune Mobilising T cells against Cancer (ImmTAC)
• Soluble T cell engagers (sTE) composed of
1. Affinity-enhanced soluble TCR = binds to pHLA complexes expressed on tumours
2. Fused to anti-CD3 scFv = engages T cells
• Facilitates antigen-specific killing of cancer cells by non-cancer-specific T cells

• TCRs are stabilised as soluble proteins through truncation of trans-membrane domains + induction of stabilising disulphide bond between 2 chains

• Increasing affinity of TCRs is very important for efficacy of ImmTACs as:
1. TCRs naturally have very low affinity for their targets (particularly cancer-specific)
2. HLA expression is down regulated in most cancers = low epitope presentation
3. Soluble TCRs cannot rely on co-receptor engagement to increase avidity
• Affinity advanced TCRs make much longer contacts with antigens  greater chance of effect

Question 6

pros and cons of ImmTacs

Answer 6

Advantages Disadvantages
Binds to pHLA so can access intracellular proteins = permits access to >90% of expressed genes
HLA-restricted
• less effective against tumours that have downregulated HLA
• therapy is restricted to patient population expressing particular HLA allele
Very high affinity for target antigen

Question 7

ImmTac key example

Answer 7

KEY EXAMPLE: TEBENTAFUSP (IMCgp100)
• Likely to be approved this year for treatment of uveal melanoma = low TMB so poorly responsive to checkpoint inhibitors
• ImmTAC consisting of TCR specific to gp100 peptide, fused to anti-CD3 scFv
• Nathan et al (2021) = phase 3 trial
- 127 patients w previously untreated metastatic uveal melanoma randomly assigned to tebentafusp or SMT (CI or dacarbazine)
- Overall survival and progression-free survival were both significantly higher in the tebentafusp group (73% vs 59% at 1 year, 31% vs 19% at 6 months)
- BUT Grade 3 and 4 adverse effects were much more common in tebentafusp group (44% vs 17%) = most occurred in first 4 weeks during dose escalation then decreased in incidence and severity with repeated dosing
- Cytokine release syndrome occurred in 89% of tebentafusp group