KARACHI (APP) - Security and Exchange Commission of Pakistan has prohibited all listed companies from amending, withdrawing, cancelling or extending buy-back offer of their own shares once approved by the members of the company. In a notification, SECP said it would be obligatory for the companies to issue tender notice within 30 days following approval of its shareholders for the purchase of shares and thereafter complete the process of buyback within 30 days of the issue of tender notice.