TSE Clearance Studies for pdFVIII: Study Methods and

Clearance Levels

TSE Advisory CommitteeSeptember 18, 2006Dorothy Scott, M.D.

Office of Blood Research and Review/CBER

TSE Safety Concerns

• Theoretically, plasma derivatives might transmit vCJD or other TSE agents

• Any such risk is thought to be very low based on the fact that no cases of vCJD have been reported worldwide in any recipients of plasma derivatives, including in the UK, where vCJD risk is greatest

• FDA seeks to assure the safety of plasma derivatives, especially pdFVIII, against risk for transmission of TSE

Importance of TSE Clearance

• Clearance of TSE agents in manufacturing of pdFVIII and other plasma derivatives has a major impact on estimated risk:– FDA risk assessment for pdFXI, 2005

(http://www.fda.gov/ohrms/dockets/ac/05/briefing/2005-4088b1.htm)

• However, standardized methods and assessment criteria for TSE clearance have not been defined.

Issue for the TSEAC

FDA seeks the advice of the Committee whether standardized methods and assessment criteria are feasible and appropriate for determining TSE clearance in the manufacturing processes for plasma-derived FVIII (pdFVIII) products.

Items for Discussion

• Feasibility and scientific value of adopting standardized methods to assess TSE clearance in manufacturing of pdFVIII products

• Whether a minimum TSE agent reduction factor might reasonably serve as an appropriate standard for demonstrating vCJD safety of pdFVIII products; and if so

• Actions FDA should consider if only lower levels of clearance can be demonstrated for a given pdFVIII product

FDA’s Evaluation of Sponsors’ Voluntary Studies of TSE Clearance

FDA discussed TSE clearance with the TSEAC in 2/03. FDA has engaged in case by case review of the following types of information on TSE clearance:

• Rationale for animal model selected• Rationale for selection of spiking preparation• Characterization of the spiking agent • Demonstration of accurately scaled-down

processes• Robust and reproducible experiments • Well-characterized assay for TSE infectivity

FDA’s Evaluation of Sponsors’ Voluntary Studies of TSE Clearance (continued)

• Estimated logs TSE clearance by processing steps

• Demonstration of “mass balance” (accounting for all input infectivity)

• Demonstration that mechanistically similar clearance steps are or are not additive

• Account for “conditioning” of infectivity where a prior step may affect physical state of TSE agent and in turn affect clearance step downstream

TSE Clearance Labeling Approvals for Plasma-derived Products

Approved Steps RF

Carimune NF precipitations 7.2nanofiltration 4.4

Panglobulin NF precipitations 7.2nanofiltration 4.4

Gamunex cloth filtration +depth filtration 6.6

Thrombate III precipitation 6.0

pdFVIII manufacturing

• Cryoprecipitation is the first step in manufacturing pdFVIII

• Other steps can include: precipitations, column purifications, some of which may result in further TSE clearance

ANTITHROMBIN III

ANION EXCHANGE CHROMATOGRAPHYHEPARIN AFFINITY CHROMATOGRAPHY

cohnfrac.org by DJFrazier 12/96

Pasteurization

(Diafiltration)(Chromatography)(Viral inactivation)(Limited proteolysis)(Lyophilization)

The Cohn-Oncley Blood Plasma Fractionation Process

Fraction IV-1 Supernate

Cryoprecipitate Supernate

Fraction I Supernate

Fraction II+III Supernate

Fraction II+IIIw Supernate

Fraction II Supernate

Fraction III Supernate

Fraction V Supernate

Fraction IV-4 Supernate

freeze, thaw

IMMUNE GLOBULINS

pH 7.425% ETOH-5 oC

pH 5.217% ETOH-6 oC

pH 7.620% ETOH

-5 oC

ALBUMIN

pH 4.610% ETOH

-2 oCFilterpH 5.1540% ETOH

-5 oCResuspend

pH 4.840% ETOH-5 oC

pH 5.840% ETOH

-5 oC

pH 5.318% ETOH

-5 oC

pH 6.8520% ETOH-5 oC

pH 7.28% ETOH-2oC

FACTOR IX COMPLEXANTIHEMOPHILIC FACTOR

Plasma

Starting material for pdFVIII (cryoprecipitate)is precipitated early in plasma fractionation schemes

Experimental Clearance of PrPTSE and Infectivity by Cryoprecipitation

Reference

Log10 Reduction Assay

Brown et al., Transfusion 38: 810-16, 1998

< 1.0 Bioassay

Foster et al., Vox Sang 78:86- 95, 2000

1.0 PrPTSE

Lee et al., Transfusion 41: 449-55, 2001

< 1.0 Bioassay

Vey et al., Biologicals 30: 187-96, 2002

< 1.0 PrPTSE

TSE Clearance Issues

• Exogenous (“spiking”) experiments– Nature of spiking material and its relevance to

blood-borne infection

• Endogenous experiments – Relevance and feasibility

• TSE strain and animal model• Output measure of infectivity reduction

– Bioassay– In vitro assays

TSE Spike Plasma

Cryoprecipitation Cryoprecipitate(FVIII)

Cryopoor Plasma Supernatant

FIX, IGIV, A1PI,Albumin, etc.

TSE Clearance Evaluation: Exogenous(“Spiking Experiment”) Model

Exogenous TSE clearance studies – form of spiking

material• Form infectious agent

– Brain homogenate, centrifuged– Ultracentrifuged (microsomal)– Caveolae-like domains

– Detergent-solubilized homogenate

– Membrane-free infectious material (e.g. fibrils)• Very insoluble• Probably NOT representative of blood infectivity

Membrane-associated

Spike form impacts clearance by precipitation

Vey et al. Biologicals 30: 187-96, 2002

Manufacturing

Process

Log10 PrPTSE Reduction (with respect to supernatant)

Microsomal Spike

Purified PrP-Scrapie Spike

Cryoppt. 0.2 2.4

8% EtOH ppt. 0.9 3.1

25% EtOH ppt. 3.1 4.0

Glycine ppt. 1.7 3.3

“Conditioning”: Detergent-treatment diminishes clearance of scrapie agent by nanofiltration

Detergent Log10 RF*

Feed solution - (8.13) ----+ (7.32) ----

Filtrate35 nM - 4.93

+ 1.61

15 nM - > 5.87+ > 4.21

10 nM + > 3.80

Tateishi et al, Biologicals 29: 17-35, 2001

* Determined by bioassay [Scrapie ME7]

“Conditioning”

• PrPTSE clearance by membrane filtration and depth filtration increases in presence of alcohol (evidence of aggregation)

Van Holten et al, Vox Sang. 85:20-24, 2003

Plasma from TSE-infected animal

Cryoprecipitation Cryoprecipitate(FVIII)

Cryopoor Plasma Supernatant

FIX, IGIV, A1PI,Albumin, etc.

TSE Clearance Evaluation: Endogenous Infection model

Endogenous TSE studies: Relevance to Blood Infectivity

• Comparison of results from endogenous and exogenous infectivity studies suggest similar reductions for some precipitations– Limited number of endogenous studies

• Endogenous infectivity characteristics in plasma– Small size– Difficult to sediment (in its native form)– Poorly aggregated– May be lipid/plasma-protein associated

Endogenous TSE Clearance Studies

• Relevance to human blood highly likely• Limited clearance can be demonstrated because

starting infectivity is low (est. 2-30 ID/ml)– Large numbers of donor and assay animals may

compensate for low titers • Recipients – volume injectible i.c. for titration: 0.02 ml mice;

0.05 ml hamsters• For 100 ml plasma: 5000 mice or 2000 hamsters

• Large animal models (sheep: Scrapie, BSE)• Experimental logistics - herd management, limited locations,

incubation time, availability• Scale-down logistics – dedicated pilot laboratories

TSE Model Selection

• TSE’s differ in resistance to inactivation• To date, clearance in plasma products

demonstrated by partitioning studies only • Few direct strain comparisons

– EtOH precipitations – clearance similar BSE, CJD, vCJD (Stenland et al)

– EtOH precipitations –Nanofiltration could be influenced if strain-related differences exist in aggregation properties (theoretical) (Vey et al)

• Strain differences for partitioning clearance experiments not demonstrated

Assays for TSE Agents

• Bioassay – limiting dilution titration into susceptible rodents

• PrPTSE proposed as surrogate marker for infectivity

• PrPTSE measured by Western Blot or Conformation-dependent immunoassay (based on binding of antibody to PrpTSE)

Rationale for Bioassay Use

• Binding assays detect PrPTSE

– Examples of infectivity without detectable PrPTSE

– Examples of PrPTSE without infectivity– Conditioning might differentially affect binding

vs. infectivity

• Binding assays (currently) not as sensitive as bioassays (limit of detection typically 2-3 logs infectivity)

Challenges in TSE Clearance interpretation – how much clearance

is “significant?”

• Viral validation (clearance) studies – typically demonstrate at least 2-3 logs greater clearance than maximum potential absolute amount of virus present

• Added clearance provides a margin of safety

TSE clearance level and safety

TSE infectivity – if present, how much might be in a single plasma unit?

800 ml (plasma unit) x 2-30 ID/ml TSE infectivity = 1600 – 24,000 ID’s (total)

= 3.2 - 4.4 log10 total infectious units estimated possible infectivity in one unit of infected plasma

• Actual infectivity might be less due to blood-brain barrier (IC/IV ID50 1 to 1 to 1 to 10 estimated [TSEAC 10/2005]), and host susceptibility

Question 1A

1. A. Please comment on the feasibility and scientific value of adopting standardized exogenous (spiking) study methods to assess TSE clearance in manufacturing of pdFVIII including the following

I) Optimal spiking material and its preparation from the standpoint of relevance to blood infectivity

II) Selection of TSE strain and animal modelIII) TSE immunoassays for PrPTSE and bioassays for

infectivityIV) Identification of manufacturing processes that might

alter TSE agent properties

Question 1B

1. B. Please comment on the feasibility and scientific value of adopting standardized endogenous study methods to assess TSE clearance in pdFVIII.

Question 2

2. Based on the available scientific knowledge, please discuss whether a minimum TSE agent reduction factor, demonstrated using an exogenous spiking model in scaled-down manufacturing experiments, might reasonably serve as an appropriate standard for demonstrating TSE safety of the products.

Question 3

3. Considering the outcome of discussion on Question 2, in cases where only lower levels of clearance can be demonstrated for a pdFVIII, should FDA consider the following:

A. Labeling that would differentiate the lower clearance products from other products with sufficient TSE clearance;

B. Recommending addition of TSE clearance steps to the manufacturing method;

C. Performance of TSE clearance experiments using endogenous infectivity models;

D. Any other actions?

TSE Clearance Labeling

• Under DESCRIPTION: “Additionally, the manufacturing process was investigated for its capacity to decrease the infectivity of an experimental agent of transmissible spongiform encephalopathy (TSE), considered as a model for the vCJD and CJD agents.”

TSE Clearance Labeling

•    Under DESCRIPTION: “Several of the individual production steps in the [product name] manufacturing process have been shown to decrease TSE infectivity of an experimental model agent. TSE reduction steps include [process] [logs], [process] [logs], etc. These studies provide reasonable assurance that low levels of CJD/vCJD agent infectivity, if present in the starting material, would be removed.