The opioid antagonist is chemically „sequestered” (isolated, see 2nd definition below) so that it can only be released under certain conditions.

sequester (se•ques•ter) (se-kwest[schwa]r) [L.; Fr. sequestrer to shut up illegally] 1. to detach or separate abnormally a small portion from the whole; see sequestration and sequestrum. 2. to isolate a constituent of a chemical system by chelation or other means; cf. sequestrant. http://www.mercksource.com/pp/us/cns/cns_hl_dorlands.jspzQzp...

Purdue is currently developing a time-release pain reliever that would contain a "sequestered antagonist." If the pill is swallowed whole, the antagonist would pass through the body without affecting the user adversely. But if the pill is crushed, the antagonist would be released and cause "an undesirable effect to the abuser." http://www.northeasttimes.com/2001/0815/oxycontin.html

Disclosed is an oral dosage form comprising (i) an opioid agonist in releasable form and (ii) a sequestered opioid antagonist which is substantially not released when the dosage form is administered intact, such that the ratio of the amount of antagonist released from said dosage form after tampering to the amount of said antagonist released from said intact dosage form is about 4:1 or greater… http://www.pharmcast.com/Patents100/Yr2004/Feb2004/022404/66...