Your question would apply to a compound with a 5-day half-life: that would be when you’d inject 600 mg if your regular mode of administration was 100 mg/day.
Incidentally, if your mode of administration was to take 500 mg every five days, then the frontload would again be by the formula, amount taken per half-life (500 mg) plus amount taken per usual administration (500 mg) so a 1000 mg frontload.
The math behind this is that at the time
of injection, once steady state has been reached, one has one half-life’s worth of material still in the system remaining from previous injections.
So for example, if your method is 500 mg taken once every 5 days, and the half-life is 5 days, when well into the cycle, immediately after an injection you will have not only the 500 mg in your system that you just injected, but also 500 mg remaining from previous injections. So to duplicate this on day 1, you need 1000 mg.
But if injecting 100 mg per day, then when well into the cycle you have 500 mg in your system from previous injections, plus 100 mg from the current one, so 600 mg total: and this is the amount you need to frontload with.
Yes, obviously the 1000 mg frontload (for the once per 5-days injection method) gives you a higher peak than the 600 mg frontload (for the daily injection method) but it also falls off to a lower level. In any case it’s the method to achieve equivalence to the steady-state condition.