Cathie Wood's ARK Innovation ETF has struggled mightily this year, down more than 50% while the S&P 500 has fallen by 18%. However, a couple of the fund's holdings did exceptionally well last month, soaring by 60%. Signify Health (SGFY) and Invitae (NVTA) were among the best-performing stocks in August, driven by very different catalysts.

Are these stocks still good buys today?

1. Signify Health

Signify Health conducts in-home health evaluations that help to ensure patients are receiving high-quality care and it works with Medicare Advantage plans. Shares of the healthcare company skyrocketed more than 60% last month on rumors that UnitedHealth Group, CVS Health, and Amazon were all looking at acquiring the business. It subsequent rose to a value above what some speculated a takeover bid might be.

The inflated price has now put Signify's stock at a forward price-to-earnings multiple of 45. By comparison, the average stock in the S&P 500 trades at a multiple of less than 18. And that's based on analyst estimates; over the trailing 12 months Signify has incurred a net loss of $338.5 million on revenue of $843.3 million.

On Monday, CVS announced that it would be acquiring Signify for approximately $8 billion in an all-cash deal worth $30.50 per share. For investors, that means there's not going to be an incentive to buy Signify's stock today. It isn't likely to fetch much more than the purchase price.

It is possible that the deal could still fall through. However, with Signify's share price being at such a high premium, this is still a stock that investors should pass on even if that does happen.

2. Invitae

Genetic testing company Invitae was another hot stock in August. It jumped 60%, but there were no big rumors swirling around its business. The only notable press release the company had last month was about its latest earnings numbers, which weren't impressive.

News of a $2.5 billion loss for the period ending June 30 (largely due to a goodwill impairment writedown of $2.3 billion) and guidance that called for revenue growth that would be in the low double digits this year isn't typically what leads to a surge in bullishness for a stock. But that's what happened with Invitae: Its share price nearly quadrupled the next day. The healthcare stock would go on to give up many of those gains, a sign that the surge was likely due to speculative reasons, but it still finished the month up an impressive 60%.

Like Signify, Invitae isn't a profitable company today. Although this past period was a particularly bad one, this marks the fourth straight quarter in which the company has finished in the red. It technically posted a profit in the prior-year period, but that was only due to a favorable change in fair value on contingent consideration, which was related to business combinations.

With a low growth rate and steep losses, Invitae looks more like a risky speculative stock at this point than a sound investment. Although in the long term it could make for a promising buy, I wouldn't invest in the business until its financials improve and profitability looks more attainable.