The other two-thirds of the outstanding notes are still redeemable in April next year.
Shanta is buying back $4.89 million of notes on a pro-rata basis based on the current holders' allotments. The record date for the buyback is May 13, with payment expected on May 17.
Shanta said in its 2018 results management's key focus was on repaying loans and maximising shareholder returns.
At the end of 2018, the company's net debt was $31.5 million, down 20% year-on-year and inclusive of the $15.1 million of convertible loan notes.
Shanta's shares (AIM:SHG) rose 2.4% Wednesday after the news to 6.96p (US9.1c), 48% higher than six months ago.