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Thrombotic thrombcytopenic purpura (TTP) (bsh 2023, BSH 2012)

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Intro


MAHA - MicroAngiopathic Haemolytic Anaemia - red cell fragmentation + thrombocytopenia

TMA - Thrombotic MicroAngiopathy - MAHA + pathological features of occlusive micro/macrovascular disease

TTP - Thrombotic Thrombocytopenic Purpura - One type of TMA, caused by ADAMTS13 deficiency

Incidence of acute idiopathic TTP:  0.8 per million of pop. (3.6 per million new hospital admissions) in UK

Untreated mortality:                         >90%

 

TTP Pathophysiology

Acquired (auto-antibodies, iTTP) or Congenital (cTTP) deficiency of ADAMTS13

ADAMTS13 – a metalloproteinase responsible for cleaving ultra large VWF multimers (ULVWF)

 

In the absence of ADAMTS13, ULWVF causes spontaneous platelet aggregation under conditions of high shear stress, e.g. microvasculature, brain, heart, kidneys.

Thrombocytopenia results from consumption within platelet-rich thrombi

Anaemia results from mechanical fragmentation of red cells through partly occluded vessels.

 

TTP Clinical Features

TTP is a diagnosis suspected from clinical features, then later confirmed by laboratory assays.

Defined as MAHA with mod-severe thrombocytopenia and associated organ dysfunction.

Presenting Clinical Features:

  • Thrombocytopenia (epistaxis, bruising, petechiae etc)

  • Central neurological (confusion, headache, paresis, seizures, apahasia, dysarthria, visual disturbance, encephalopathy, coma

  • Fever >37.5oC

  • Jaundice (unconjugated hyperbilirubinaemia)

  • Renal impariment (proteinuria, microhaematuria)

  • Cardiac (chest pain, heart failure, hypotension, cardiac arrest)

  • GI Tract (abdominal pain, pancreatitis, ischaemia)

  • Non-specific (pallor, jaundice, fatigue, arthralgia, myalgia)

Note:

  • 33% No Neurological Signs

  • Fever and renal impairment may not be prominent

 

TTP Ix

 

FBC: Median plt count 10-30 & Hb 80-100 at diagnosis

Film: thrombocytopenia, schistocytes/fragments

Haemolysis markers: Low haptoglobins, Raised Retic, Raised LDH, Normal DAT

Coag screen: Normal

Trop T : Raised in 50%, poor prognostic sign

Others: B12/Folate/Iron

 

ADAMTS13 Assays

  • <10% confirms the diagnosis

  • (ISTH include an equivocal range of 10-20% when congenital TTP should be considered)

  • TTP vs HUS – A level of <5% has 90% specificity for TTP

  • Activity and Autoantibody assays available (see bottom of this page).

 

Ix to exclude other causes of MAHA/TMA

 

Pregnancy Test

Infection: HIV, Hep A/B/C, Stool culture for E.coli

AutoImmune: TFT, Glucose, Autoantibody screen

Cardiac: ECG, Echo

Neuro: CT/MRI Brain

Malignancy: CT CAP, tumour markers

 

Differential Diagnosis of MAHA/TMA

 

Acquired TTP

Congenital TTP

  • Biallelic recessive variants in ADAMTS13 gene. Presents at any age. Often asymptomatic until a precipitating event sends a chronically low ADAMTS13 even lower.

Disseminated Intravascular Coagulation (DIC)

Pregnancy

  • HELLP, Eclampsia, AFLP

  • Diagnostically difficult. Pregnancy may be precipitant for 5-25% of TTP cases

Haemolytic Uraemic Syndrome (HUS)

  • E. coli shiga toxins

Atypical HUS (aHUS)

  • Excess activation of alternate complement pathway

  • May be genetic, acquired autoantibody or idiopathic

Drugs

  • 15% of MAHA cases

  • TTP - Quinine, tacrolimus, simvastatin, interferon, OCP, trimethoprim

  • HUS – Gemcitabine, bleomycin, mitomycin-C

Malignant Hypertension

Malignancy

  • Adenocarcinoma especially. Early or late stage disease. ADAMTS13 not low.

Post-HSCT

  • Lack of ADAMTS13 deficiency, poor response to PLEX. Anecdotal use of defibrotide

Infection

  • CMV, Adenovirus, Herpes Simplex, Hep B, Hep C, Meningococcus, Pneumococcus, Fungal

  • HIV – may be presenting feature. ADAMTS13 <5% associated with a relatively higher CD4 count

Autoimmune

  • Autoimmune Haemolysis, Lupus nephritis, Scleroderma, Evans Syndrome, Vasculitis

Pancreatitis

Malignant hyperthermia / heat shock

Severe aortic stenosis / paravalvular leaks

Catastrophic Antiphospholipid Syndrome (CAPS)

diagnostic scoring systems

Scores may aid identification of TMA patients who are most likely to have TTP. However, they have not been prospectively validated and sensitivity/specificty may decrease with patient age.

PLASMIC Score

French Score

Treatment of Acute TTP

 

1. Start Plasma Exchange (PLEX) within 4-8 hours of suspected diagnosis

Removes offending antibody and VWF Multimers

Replaces ADAMTS13

Delivery:

  • 1.5x plasma volume exchange daily for 3 days, then re-assess

  • Continue PLEX until plt count >150 (Complete remission), then stop (provided on caplacizumab)

  • Use SD-FFP to reduce risk of TTI & adverse immune responses.

  • (Note: MB-FFP associated with longer hospital stay and greater number of exchanges)

  • (Rock et al NEJM 1991 for PLEX vs Plasma transfusion survival)

2. Caplacizumab

  • First dose should be given IV pre-plasma exchange (but do not delay PLEX for this)

  • See next section for details

3. Start steroids

Stop production of antibody

  • 1g IV Methylprednisolone or 1mg/kg PO Prednisolone

  • Dose and length of treatment varies between centres

  • Expect an exacerbation of symptoms+thrombocytopenia 7-10 days after steroids

4. Rituximab (Off license)

Stop production of antibody

  • Dosed every 3-4 days

  • Reduces No. of PLEX, length of stay and time to CR

  • Median 10 days to effect —> better outcomes if given early

  • Some centres give to all patients, some only to those with cardiac/neuro complications

Prolongs time to relapse but no difference in overall relapse rate vs no ritux (Doyle 2023)

5. Supportive Care

Avoid platelet transfusion

PPI

Folic Acid

Prophylactic LMWH once plt count >50 (SD-FFP deficient in protein S)

(Avoid aspirin until caplacizumab complete)

Red cell transfusion if required

Consider Hep B vaccination

6. Follow-up

Life-long term follow-up to manage physical and psychological effects of treatment and monitor for relapse

Screen for memory, neurological, cardiac and renal impairment

Assess for anxiety/depression

7. Difficult scenarios

In cases refractory to standard treatments a wide range of immunosuppressive / immunomodulatory therapies have been used, including:

  • MMF, Ciclosporin, Azathioprine, Daratunumab, Bortezomib, Vincristine, Cyclophosphamide

  • Splenectomy


caplacizumab


Pharmacology

  • 28kD bivalent camelid Nanobody, targeting A1 region of VWF

  • (Camelid’s produce heavy-chain only antibodies which are stable and fully functional)

  • Theory: release platelets from their ULVWF-bound state, returning them to the circulation

Benefits

  • Reduces duration of thrombocytopenia, exacerbations, refractory disease, admission duration, PEX procedures and volume of plasma used.

  • It does not address the underlying disease.

Side Effects

  • Reduced VWF activity —> bleeding. Severe bleeding rare but VWF concentrates can be considered.

Dosing

  • First dose should be given IV pre-plasma exchange (but do not delay PLEX for this)

  • 10mg SC after 1st exchange then 10mg daily SC for 30 days

  • If ADAMTS13 <10% at day 30 —> continue to 60 days

Is Plasma Exchange still needed? (2022)

TITAN Study NEJM 2016, Phase II

  • 30 days capla for TTP diagnosed based on clinical basis (ie pre-ADAMTS13 level result)

  • Could not recruit, stopped early, ?Poor study design (Drug have to be given before PLEX)

  • Despite this, did show reduced time to platelet recovery, reduced exacerbation, but higher relapse

  • Conclusion: Bridging therapy that reduces microthrombi but does not treat the underlying disease

HERCULES NEJM 2019, Phase III

  • Randomised, double-blind, placebo-controlled, multi-national study

  • Plasma exchange allowed prior to starting drug

  • Able to extend drug beyond 30 days if ADAMTS13 level still low, or if exacerbation occurs.

  • Reduces No. of exchanges, volume of plasma, No. of ICU days, No. of days in hospital

  • SE: More bleeding, but minor sites and severity (nose, gums) – theory: organs don’t bleed as full of microthrombi, skin and epithelium has the space to bleed. Aim to manage symptoms, but do not stop caplacizumab.

Further Details of Plasma Exchange

 

£3,000 per exchange (including cost of SD-FFP)

Two methods of separation - Centrifugal or Filtration

Removes ultra large vWF, replaces ADAMTS13 and removes inhibitor (e.g. antibody)

FFP contains around 1 unit ADAMTS13 per 1ml/kg. So 15ml/kg --> Raise ADAMTS13 by up to 15% (however rapidly consumed or deactivated by antibody)

Replace 1-1.5x plasma volume & takes 110-150 minutes per exchange

Flow rate around 120ml/min through machine

Centrifuge at 2400 rpm

ACD-A anticoagulant

Perform daily until platelet count >150 on 3 consecutive days, then wean off.

Specific Scenarios

  

Relapse

  • Defined as an episode of acute TTP occurring >30 days after remission

  • Can be averted by ADAMTS13 monitoring and pre-emptive rituximab

  • Relapse rate: 30% at 3 years

  • 40% pts w/ ADAMTS13 relapse have symptoms b4 onset of clinical TTP - usually headache/lethargy

  • Frequency of relapse does not affect response rate to Rituximab

Congenital TTP (syn. Upshaw-Schulman Syndrome)

  • Consider long-term prophylaxis – SD-FFP transfusion every 10-20 days

  • Alternative: ‘8Y’  - a intermediate purity FVIII concentrate containing ADAMTS13

  • Recombinant ADAMTS13 available on compassionate access schemes (2022). Blood 2026 - Phase 3 trial, 48 patients, rADAMTS13 vs plasma-based therapy for 6 months, then switched over. Offers several clinical benefits.

  • Supplement during pregnancy

Pregnancy-Associated TTP

  • cTTP and iTTP can both present for first time in pregnancy. >40% risk of fetal loss

  • DDx: HELLP, Eclampsia, aHUS

  • Treate with PEX and steroids. Caplacizumab not currently recommended in pregnancy. Small molecule, expected to be able to cross placenta.

  • Fetal thrombocytopenia is not expected

  • Monitor levels for rest of current pregnancy and throughout future pregnancies

HIV-associated TTP

  • Wide range of causes for TMA in HIV

  • True TTP in HIV responds to PEX, steroids, caplacizumab and HAART

 

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lab notes on adamts13 assays (BSH2026)

Sample Collection/Preparation/Transport

  • Citrated whole blood, which must be transported to testing laboratory at ambient room temperature for immediate testing.

  • If testing is going to be delayed, centrifuge and freeze sample, then transport to testing lab on dry ice

2nd Generation ADAMTS13 Activity Assays (commonly used)

Principle: testing the ability of patient plasma to cleave an external source of truncated VWF (eg VWF73)

ADAMTS13 <10% by FRET assay is 97% specific, 100% sensitive for a diagnosis of TTP

Commonly used quantitative methods:

  • Manual Indirect Assays - ELISA, turn-around time ~5 hours. - e.g. TECHNOZYM

  • Automated Indirect Assays - CLEIA (e.g. HemosIL AcuStar)

  • Automated Direct Assays - FRET (e.g. TECHNOFLUOR)

ADAMTS13 Antibody Assays

Detection of anti-ADAMTS13 IgG antibodies is consistent with a diagnosis of iTTP

Required for all cases of ADAMTS13 <10%. Can also be used to detect development of inhibitors in cTTP patients on FFP or Factor concentrates.

The antibody level correlates with mortality, length of stay and no. of plasma exchanges

Commonly used quantitative methods:

  • ELISA - detects both neutralsiing and non-neutralising antibodies (e.g. Technozym)

  • Bethesda assay - only detects neutralising antibodies

Other Tests (Infrequently used)

ADAMTS13 Antigen Assays - May be normal in TTP, uninformative without activity assay. May have a role in decisions on discontinuing caplazicumab but this is a research question. Available methods: Quantitative ELISA

ADAMTS13 Conformation Assay - assesses the open vs closed conformation of the ADAMTS13 structure. May predict risk of relapse. Research tool at present.

Semi-quantitative screening assays (eg Technoclone) have high sensitivity and negative predictive low, but there have been reports of false negatives and current UK advice is not to rely on these tests.

1st Generation Activity assays - Use modified, full length VWF and turn around time is several days.

SDS Gel Electrophoresis – incubate vWF with plasma and measure the drop in multimer size compared to diluted normal plasmas. Complicated and time consuming.

SDS PAGE & Western blotting – similar to above

example of ADAMTS13 Indirect Activity ELISA

A microtitre plate is coated with anti-GST, subsequently bound to VWF73.

A source of ADAMTS-13 is then added (Calibrator, Control or Patient).

Cleavage of the substrate allows HRP antibody to bind to the remaining fragment --> colour change

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 example of ADAMTS-13 Inhibitor ELISA

 

A microtire plate is coated with recombinant ADAMTS-13.

When incubated with a source of ADAMTS-13 inhibitor (Claibrator, Control, Patient), the antibody will bind to ADAMTS-13.

Addition of an HRP antibody will produce a colour change

(Simple alternative: 50:50 mix with normal plasma)

adamst 2.png