Is zakat due annually on capital and profits, or on profits only, given that the borrower has not yet paid all installments of his loan, and what he has left monthly after paying the installments is less than the remaining debt?

If you participated in a medical laboratory with the intention of benefiting from its income, and not to sell your share, then there is no zakat on the principal capital, but rather on the profits if they reach the nisab and a hawl (full lunar year) has passed on them. In this case, you pay 2.5%. You deduct any remaining debt you owe from the money before calculating zakat, and then pay zakat on the remainder if it does not fall below the nisab, unless you possess an asset for personal use that is surplus to your needs and can be used to offset the debt. In that case, you pay zakat on all the profits without deducting the debt.