EXHIBIT 4.15


 


DESCRIPTION OF SECURITIES 


 


The foregoing description is intended as a summary
and is qualified in its entirety by reference to our certificate of incorporation, as amended (the “Certificate of Incorporation”)
and our amended and restated by-laws, as amended (the “By-laws”) as currently in effect, copies of which are filed as exhibits
to this Annual Report on Form 10-K and are incorporated by reference herein.


 


Authorized Capital Stock


 


As of December 31, 2020, we have authorized 1,050,000,000
shares of capital stock, par value $0.001 per share, of which 1,000,000,000 are shares of common stock and 50,000,000 are shares of “blank
check” preferred stock. As of December 31, 2020, there were 17,532,893 shares of common stock issued and outstanding. The authorized
and unissued shares of common stock and the authorized and undesignated shares of preferred stock are available for issuance without further
action by our stockholders, unless such action is required by applicable law or the rules of any stock exchange on which our securities
may be listed.


 


On August 10, 2020, we filed an amendment of our Certificate
of Incorporation to effect a 1-for-30 reverse stock split of our common stock (the “Reverse Stock Split”). The Reverse Stock
Split became effective as of 5:00 p.m. Eastern Time on August 10, 2020 (the “Effective Time”) and our common stock began trading
on a split-adjusted basis when the market opened on August 11, 2020. As a result of the Reverse Stock Split, every 30 shares of our pre-Reverse
Stock Split common stock were combined and reclassified into one share of our common stock. No fractional shares were issued in connection
with the Reverse Stock Split, and cash paid to stockholders for potential fractional shares was insignificant. As of the Effective Time,
we adjusted and proportionately decreased the number of shares of our common stock reserved for issuance upon exercise of, and adjusted
and proportionately increased the exercise price of, all options and warrants and other rights to acquire our common stock. In addition,
as of the Effective Time, we adjusted and proportionately decreased the total number of shares of our common stock that may be the subject
of the future grants under our stock plans.


 


Common Stock


 


The holders of our common stock are entitled to one
vote per share. Our Certificate of Incorporation does not provide for cumulative voting. Our directors are divided into three classes.
At each annual meeting of stockholders, directors elected to succeed those directors whose terms expire are elected for a term of office
to expire at the third succeeding annual meeting of stockholders after their election. The holders of our common stock are entitled to
receive ratably such dividends, if any, as may be declared by our board of directors out of legally available funds. Upon liquidation,
dissolution or winding-up, the holders of our common stock are entitled to share ratably in all assets that are legally available for
distribution. There are no preemptive, subscription, conversion rights, redemption or sinking fund provisions regarding the common stock.
The rights, preferences and privileges of holders of our common stock are subject to, and may be adversely affected by, the rights of
the holders of any series of preferred stock, which may be designated solely by action of our board of directors and issued in the future.


 


The transfer agent and registrar for our common stock
is Action Stock Transfer Corporation. The transfer agent’s address is 2469 East Fort Union Blvd., Suite 214, Salt Lake City, UT
84121. Our common stock is listed on the NYSE American under the symbol “ATNM.”


 


Preferred Stock


 


The board of directors is authorized, subject to any
limitations prescribed by law, without further vote or action by the stockholders, to issue from time to time up to 50,000,000 shares
of preferred stock in one or more series. Each such series of preferred stock shall have such number of shares, designations, preferences,
voting powers, qualifications, and special or relative rights or privileges as shall be determined by the board of directors, which may
include, among others, dividend rights, voting rights, liquidation preferences, conversion rights and preemptive rights. Issuance of preferred
stock by our board of directors may result in such shares having dividend and/or liquidation preferences senior to the rights of the holders
of our common stock and could dilute the voting rights of the holders of our common stock.


 


 



 


Prior to the issuance of shares of each series of
preferred stock, the board of directors is required by the Delaware General Corporation Law and our Certificate of Incorporation to adopt
resolutions and file a certificate of designation with the Secretary of State of the State of Delaware. The certificate of designation
fixes for each class or series the designations, powers, preferences, rights, qualifications, limitations and restrictions, including,
but not limited to, some or all of the following:


 


	 	●	the number of shares constituting that series and the distinctive designation of that series, which number may be increased or decreased (but not below the number of shares then outstanding) from time to time by action of the board of directors;
	 	●	the dividend rate and the manner and frequency of payment of dividends on the shares of that series, whether dividends will be cumulative, and, if so, from which date;
	 	●	whether that series will have voting rights, in addition to any voting rights provided by law, and, if so, the terms of such voting rights;
	 	●	whether that series will have conversion privileges, and, if so, the terms and conditions of such conversion, including provision for adjustment of the conversion rate in such events as the board of directors may determine;
	 	●	whether or not the shares of that series will be redeemable, and, if so, the terms and conditions of such redemption;
	 	●	whether that series will have a sinking fund for the redemption or purchase of shares of that series, and, if so, the terms and amount of such sinking fund;
	 	●	whether or not the shares of the series will have priority over or be on a parity with or be junior to the shares of any other series or class in any respect;
	 	●	the rights of the shares of that series in the event of voluntary or involuntary liquidation, dissolution or winding up of the corporation, and the relative rights or priority, if any, of payment of shares of that series; and
	 	●	any other relative rights, preferences and limitations of that series.


 


Once designated by our board of directors, each series
of preferred stock may have specific financial and other terms.


 


Anti-Takeover Law and Provisions of our Certificate
of Incorporation and By-laws


 


Provisions of our Certificate of Incorporation and
By-laws may delay or discourage transactions involving an actual or potential change in our control or change in our management, including
transactions in which stockholders might otherwise receive a premium for their shares, or transactions that our stockholders might otherwise
deem to be in their best interests. Therefore, these provisions could adversely affect the price of our common stock. Among other things,
our Certificate of Incorporation and By-laws:


 


	 	●	permits our board of directors to issue up to 50,000,000 shares of preferred stock, without further action by the stockholders, with any rights, preferences and privileges as they may designate, including the right to approve an acquisition or other change in control;
	 	●	provide that all vacancies, including newly created directorships, may, except as otherwise required by law, be filled by the affirmative vote of a majority of directors in office;
	 	●	divide our board of directors into three classes, with each class serving staggered three-year terms;
	 	●	do not provide for cumulative voting rights (therefore allowing the holders of a majority of the shares of common stock entitled to vote in any election of directors to elect all of the directors standing for election, if they should so choose);
	 	●	provide that special meetings of our stockholders may be called only by our board of directors, chairman or chief executive officer; and
	 	●	provide advance notice provisions with which a stockholder who wishes to nominate a director or propose other business to be considered at a stockholder meeting must comply.