Risk Wise

[

] 49

Combination of human monoclonal antibodies

for post-exposure prophylaxis of rabies:

leading the way in antiviral innovation

Jaap Goudsmit and Fons UytdeHaag, Crucell Holland BV, Leiden, The Netherlands

he human immune system reacts when it encounters a

virus with a polyclonal immunoglobulin response specific

for that particular virus type. Most of these virus-specific

antibodies are unable to keep the virus from infecting cells that

carry the receptor for it, and are therefore able to replicate the

virus. Some of these virus-specific antibodies coat the virus with

a layer of antibodies. These cover the envelope of the virus and

make it impossible for it to reach the cells in the human body

that are able to multiply it. If such antibodies are effective, a

limited and abortive infection occurs at the port of entry, but

little or no virus production ensues. Such antibodies are present

in a small but effective proportion of plasma in vaccinated indi-

viduals, and in individuals who cleared a particular infection.

In the early days of antiviral therapy, hyperimmune plasma prepara-

tions taken from vaccinated individuals or from people who cleared

the infection were made and used to prevent and/or treat viral infec-

tions. Success with prevention of infection or disease was achieved for

hepatitis A and B viruses, for RSV, for CMV, for VZV, measles, vaccinia

and for rabies. Treatment success was obtained – albeit in some cases

limited – for CMV, parvovirus, for enteroviruses and vaccinia. Most

of these preparations were made from human plasma with the risk

of transmission of viruses, like hepatitis B and C and HIV, that reside

in blood. Experts uniformly promote this method, as long as the tech-

nology to isolate and produce monoclonal antibodies to be used to

replace immunoglobulins comes into existence. Crucell’s monoclonal

mixture against rabies is the farthest ahead to reach this goal.

Post-exposure prophylaxis of human rabies

The original idea of Louis Pasteur was that following a bite by a rabid

dog, fox or bat, vaccination could prevent the development of symp-

tomatic rabies. In 1954 Marcel Baltazard, director of the Pasteur of

Iran institute, published his findings of post-exposure prophylaxis

exclusively using the rabies vaccine. Baltazard considered the results

to be disastrous. Of the 186 individuals bitten in the head, 53 (28 per

cent) developed rabies and died. From these data it may be assumed

that rabies vaccination prevents about 50 per cent of rabies cases.

In 1954, Baltazard did a key experiment using a rabbit immunoglob-

ulin preparation showing that the combination of antirabies

hyperimmune serum with rabies vaccine was able to completely

prevent rabies. Eighteen people were bitten in the head by a rabid wolf

with proven rabies infection. Five received two inoculations of serum

plus vaccine and all survived the attack; seven received one inocula-

tion of serum and vaccine, and six of these survived; of

the five individuals that only received vaccine, three

succumbed to rabies. The one child whose scalp was

bitten, and who received multiple local and systemic inoc-

ulations of serum plus vaccine, survived. Combined, these

data underscored the importance of administering rabies

immunoglobulin besides vaccine.

Karl Habel, working at the National Institutes of Health

in the US, developed the first potency test for rabies

vaccine and antibodies. Using the so-called Habel-test for

potency, he showed convincingly in the 1960s that one

could use a serum from a species that was identical to the

challenged recipient or a serum from a non-matched

species with the same effect. This test involved mice

infected with the rabies virus. Later, a correlation of

protection was established by the World Health

Organization (WHO) using an in vitro test, the so-called

modified rapid fluorescent focus inhibition neutraliza-

tion test. This assay has allowed later studies to establish

the protective dose of monoclonal antibodies.

The road to replacing immunoglobulin

preparations by monoclonal antibody or

combination of antibodies

Mouse monoclonal antibodies, as well as human mono-

clonal antibodies, have been shown to protect rodents from

a lethal rabies virus challenge. One of the most potent of

human antibodies neutralizing rabies was discovered and

described by Bernhard Dietzschold of Thomas Jefferson

University in Philadelphia. He subsequently included this

in a cocktail of three antibodies, SOJA, SOJB and SO57.

This cocktail protected mice effectively from a lethal dose

of rabies virus. All three antibodies were subsequently

licensed and entered the 2004-2005 rabies programme.

From the onset, criteria were established for a mono-

clonal antibody product; a single antibody was at no point

considered because of the risk of escape from one anti-

body to another. After consulting the leading world experts

on rabies it became clear that two antibodies should be

included in an effective antibody mixture, which had to be

of high quality. The inclusion criteria formulated were: the

MoAbs should target distinct, non-overlapping epitopes;

and preferably should not compete for binding to rabies