Filed Pursuant to Rule 424(b)(5)
Registration No. 333-230723
AMENDMENT NO. 1 DATED June 1, 2020
To Prospectus
Supplement dated March 27, 2020
(to Prospectus dated April 12, 2019)
$50,000,000
Common Stock
This
Amendment No. 1 to Prospectus Supplement, or this amendment, amends our prospectus supplement dated March 27, 2020, or the prospectus supplement. This amendment should be read in conjunction with the prospectus supplement and the
prospectus dated April 12, 2019, each of which are to be delivered with this amendment. This amendment amends only those sections of the prospectus supplement listed in this amendment; all other sections of the prospectus supplement remain as
is.
On March 27, 2020, we entered into an Equity Distribution Agreement with JMP Securities LLC, or JMP Securities, relating to
shares of our common stock offered by the prospectus supplement, as amended by this amendment. In accordance with the terms of the Equity Distribution Agreement, we may offer and sell shares of our common stock having an aggregate offering price of
up to $50.0 million from time to time through JMP Securities as the sales agent. As of the date of this amendment, we have sold 164,900 shares of our common stock for an aggregate of $579,546 pursuant to the Equity Distribution Agreement.
Our common stock is currently listed on the Nasdaq Global Market under the symbol ALT. On May 29, 2020, the last reported sale
price of our common stock was $9.28 per share. Based on 20,111,698 shares of outstanding common stock as of May 29, 2020 of which 19,751,571 were held by non-affiliates as of such date, and a per share price of $9.28 as of May 29, 2020,
the last reported sales price of our common stock on the Nasdaq Global Market on such date, the aggregate market value of our outstanding common stock held by non-affiliates is approximately $183 million.
Sales of our common stock, if any, under the prospectus supplement, as amended by this amendment, may be made in sales deemed to be at
the market offerings as defined in Rule 415(a)(4) promulgated under the Securities Act of 1933, as amended, or the Securities Act. JMP Securities will act as sales agent and use commercially reasonable efforts to sell on our behalf all of the
shares of common stock requested to be sold by us, consistent with its normal trading and sales practices, on mutually agreed terms between JMP Securities and us. There is no arrangement for funds to be received in any escrow, trust or similar
arrangement.
JMP Securities will be entitled to compensation at a commission rate equal to 3.0% of the aggregate gross sales. In
connection with its sale of common stock on our behalf, JMP Securities will be deemed to be an underwriter within the meaning of the Securities Act, and the compensation of JMP Securities will be deemed to be underwriting commissions or
discounts.
Investing in our common stock involves a high degree of risk. You should review carefully the risks and uncertainties
described under the heading Risk Factors on page S-4 of this amendment, and under similar headings in the prospectus supplement dated March 27, 2020 and in
the documents that are incorporated by reference into the prospectus supplement and the accompanying prospectus.
Neither the
Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities, or passed upon the adequacy or accuracy of this amendment, the prospectus supplement and the accompanying prospectus. Any
representation to the contrary is a criminal offense.
JMP Securities
The date of this Amendment No. 1
to Prospectus Supplement is June 1, 2020.