Calling HttpRequest with Drush produce PHP fatal error

Question

I created a drush command which call a function which send emails. When I'm calling this method with the devel help module, the function is called correctly and my emails are sent. But when I tried to call it from a drush command I have this error below :

Drush command terminated abnormally due to an unrecoverable error.
[error] Error: Class 'HttpRequest' not found

Here is how I instanciate my HttpRequest class :

// Configuration client Http
$clientHttp = new HttpRequest($url, HttpRequest::METH_POST);

I am working on my local environment with WAMP (PHP 5.2.11).

I will be very happy if you can give me some advice, thanks !